Learn Hardcore Japanese: densya de ha ookii heddohon yori, tiisai iyahon no hou ga tukaiyasui. - On the train, small earphones are easier to use than big headphones.

Place where an action happens
- 学校で勉強する = to study at school
- 公園で遊ぶ = to play in the park
Means / method
- 電車で行く = to go by train
- バスで帰る = to return by bus

In your sentence:

電車では大きいヘッドホンより、小さいイヤホンのほうが使いやすい。

the meaning is “on/in trains (as a place/situation)”, not “by train (as transportation).”
So here 電車で means “on the train / in trains” = the place where you use headphones/earphones.

Why is it 電車で は? What does adding は to make 電車では do?

では here is で + は:

で = at / in (place of the action)
は = topic marker (“as for …”, “in the case of …”)

So:

電車で = on trains, in trains (location of the action)
電車では = as for when you are on trains / in the context of trains

Nuance:

電車で大きいヘッドホンより、小さいイヤホンのほうが使いやすい。
= On trains, small earphones are easier to use (neutral statement of place).
電車では大きいヘッドホンより、小さいイヤホンのほうが使いやすい。
= As for on trains, small earphones are easier to use (suggests a contrast with other situations, e.g. “at home it might be different, but on trains…”).

So では makes “on trains” the topic/situation you’re talking about, with a slight sense of “in that case / in that setting.”

How does the pattern AよりBのほうがX work? Which thing is “easier to use” in this sentence?

The pattern is:

A より B のほうが X
= B is more X than A.

A = the thing you compare from (the “less X” thing)
B = the thing you compare to (the “more X” thing)
X = the adjective/description

In your sentence:

大きいヘッドホンより、小さいイヤホンのほうが使いやすい。

A = 大きいヘッドホン (big headphones)
B = 小さいイヤホン (small earphones)
X = 使いやすい (easy to use)

So it means:

Small earphones are easier to use than big headphones.

General example:

犬より猫のほうが好きです。
= I like cats more than dogs.

Why do we need のほう after 小さいイヤホン? Why not just say 小さいイヤホンが使いやすい?

You can say:

大きいヘッドホンより、小さいイヤホンが使いやすい。

It’s grammatically correct and understandable.

However, Bのほうが is a very common and natural way to explicitly show “this option is preferable / comes out ahead in this comparison.”

Nuance:

小さいイヤホンが使いやすい
= Small earphones are easy to use. (Here, the “than big headphones” part must be inferred from context or from preceding より.)
小さいイヤホンのほうが使いやすい
= The small-earphones side/option is easier to use (than big headphones).
This more clearly frames it as “between these two, this one wins.”

So のほうが just makes the comparative choice more explicit and is very typical in AよりBのほうが〜 sentences.

What exactly is the の in 小さいイヤホンのほう? What is it referring to?

Literally, ほう (方) means “direction / side / way.”
X のほう originally meant “the side of X,” “the X side/option.”

In 小さいイヤホンのほう:

小さいイヤホン = small earphones
の = links the noun phrase to ほう
ほう(方) = side / alternative / choice

So very literally: “the small-earphones side (option)”.

In modern usage, Xのほうが〜 has become almost a set comparative pattern, and you don’t usually think of a specific omitted word. You can just learn Xのほうが〜 as “X is more ~ (than the other one).”

Why is it のほうが and not のほうは? Can I say 小さいイヤホンのほうは使いやすい?

In your sentence:

小さいイヤホンのほうが使いやすい。

が marks the item that satisfies the predicate (“is easy to use”) and is also the one that “wins” in the comparison. This fits the AよりBのほうがX structure.

You can say:

小さいイヤホンのほうは使いやすい。

but the nuance changes:

のほうが:
- Pure comparative:
- “Between A and B, B is more X.”
- It directly answers “which is easier to use?”
のほうは:
- Topical: “as for the small-earphones side…”
- Often implies contrast with something else that will be mentioned or is in mind:
  - 小さいイヤホンのほうは使いやすいけど、音はあまり良くない。
    = The small-earphones side is easy to use, but the sound isn’t so good.

In AよりBのほうがX, が is the standard, natural choice.

Why does より come after 大きいヘッドホン? Could I move it somewhere else?

より attaches directly to the thing you are comparing from (the “less X” item).

Pattern:

[comparison base] + より, [comparison target] のほうが X

So:

大きいヘッドホンより = than big headphones
小さいイヤホンのほうが使いやすい = small earphones are easier to use

You can move the whole Aより〜 phrase around a bit in the sentence, but より itself must stay attached to A:

電車では、大きいヘッドホンより小さいイヤホンのほうが使いやすい。
電車では、小さいイヤホンのほうが大きいヘッドホンより使いやすい。

Both are acceptable.
What you cannot do is detach より from its noun or attach it to the wrong one, for example:

✕ 大きいヘッドホン小さいイヤホンよりのほうが… (ungrammatical)
✕ 小さいイヤホンよりのほうが大きいヘッドホンは使いやすい。 (the より is attached wrongly and the meaning becomes confused)

As long as より is right after the thing you’re saying “than X” about, you’re fine.

What does 使いやすい literally mean? How does this 〜やすい form work?

使いやすい comes from:

使う (to use) → verb stem: 使い
やすい (easy to ~)

Verb-stem + やすい = “easy to do (that verb).”

So:

使う → 使いやすい = easy to use
読む → 読みやすい = easy to read
見る → 見やすい = easy to see
聞く → 聞きやすい = easy to hear

Opposite form:

Verb-stem + にくい = hard/difficult to do
- 使いにくい = hard to use
- 読みにくい = hard to read

So 使いやすい literally means “easy-to-use,” and that’s exactly how it’s used in Japanese: both for physical ease and convenience.

Could I say 電車の中では instead of 電車では? Is there a difference?

Yes, you can say:

電車の中では大きいヘッドホンより、小さいイヤホンのほうが使いやすい。

Differences in nuance:

電車では
- “On trains / in the situation of trains” (fairly broad)
- Includes the general context of being on a train.
電車の中では
- More literally “inside the train”
- Emphasizes the physical interior space of the train (crowded, limited space, etc.)

In many contexts, they will feel almost the same, but 電車の中では makes you picture the inside of the carriage more concretely.

Is 大きいヘッドホン / 小さいイヤホン natural Japanese, or should I say something like 大きな / 小さな?

Both are natural, but with slightly different flavors:

大きいヘッドホン / 小さいイヤホン
- Very normal, neutral descriptions: “big headphones / small earphones.”
- Perfectly fine in everyday speech and writing.
大きなヘッドホン / 小さなイヤホン
- 大きな / 小さな are more literary / slightly formal or descriptive.
- You see them more often in written descriptions, stories, or when you want a bit of stylistic color.

In this casual, general statement, 大きいヘッドホン and 小さいイヤホン are completely natural choices.