beruto wo sinai to, kono zubon ha sukosi ookii desu.

Here, ベルトをしない means not wear a belt, and と means if/when.

So ベルトをしないと means:

• if I don’t wear a belt
• unless I wear a belt
• without a belt

In this sentence, it connects to the result in the second half: if no belt is worn, the pants feel a little big.

Yes, it often does, so this is a very common point of confusion.

For example:

• 行かないと。 = I have to go.
• 勉強しないと。 = I have to study.

That usage is actually a shortened form of expressions like:

• 〜ないといけない
• 〜ないとだめ

But in your sentence, 〜ないと is not being used that way.
It is a normal conditional:

• ベルトをしないと、このズボンは少し大きいです。
• If I don’t wear a belt, these pants are a little big.

So whether 〜ないと means if not or have to depends on what comes after it.

Because ベルトをする is a common expression meaning to wear a belt.

So:

• ベルトをする = wear a belt
• ベルトをしない = not wear a belt

The particle を marks the thing involved in the action.

This may feel strange if you are used to thinking of する as just do, but in Japanese, 〜をする is also used with some wearable items, such as:

• ネクタイをする = wear a tie
• マスクをする = wear a mask

は marks このズボン as the topic: as for these pants...

So the sentence is structured like:

• As for these pants, if I don’t wear a belt, they are a little big.

Using は is natural because the sentence is making a comment about the pants.

It can also give a slight contrastive feeling, like:

• These pants, unlike some others, are a little big without a belt.

Because 大きい is an い-adjective.

With い-adjectives, you keep the final い and then add です for politeness:

• 大きい = big
• 大きいです = is big

So:

• 大きです ❌
• 大きいです ✅

This is different from な-adjectives, where you do not have that final い pattern.

少し means a little or slightly.

So 少し大きいです means:

• is a little big
• is slightly big

It softens the statement.
Without 少し, 大きいです would sound more direct: it’s big.

Because Japanese and English handle this word differently.

In Japanese:

• ズボン usually refers to one pair of pants/trousers

In English:

• pants is grammatically plural
• so we usually say these pants, not this pants

So:

• このズボン = these pants
• more literally: this pair of pants

Nothing is wrong with the Japanese here; it is just a difference between the two languages.

The subject is omitted because Japanese often leaves it out when it is clear from context.

So the sentence does not explicitly say:

• I
• you
• he
• she

Usually, the listener understands it from the situation.

In many contexts, this would naturally mean something like:

• If I don’t wear a belt, these pants are a little big.

But it could also refer to someone else if the conversation makes that clear.

Yes, other conditionals are possible, but と is very natural here.

と often expresses a result that is regular, natural, or expected:

• ベルトをしないと、このズボンは少し大きいです。
• If I don’t wear a belt, these pants are a little big.

This sounds like a straightforward fact about how the pants fit.

Other forms are possible, but the nuance changes:

• ベルトをしなかったら... = if I didn’t wear / if I don’t wear...
  More situational or hypothetical.
• ベルトをしなければ... = if I do not wear...
  More formal or stiff.
• ベルトをしないなら... = if you’re not going to wear a belt...
  Feels more like reacting to a stated condition.

So と works well because the second part is a natural consequence.

大きい is grammatically fine and understandable. It means the pants are big or too big.

But in real life, if you mean the waist feels loose, Japanese speakers often say ゆるい:

• このズボンは少しゆるいです。 = These pants are a little loose.

So:

• 大きい focuses more on overall size
• ゆるい focuses more on looseness of fit

Because the sentence mentions needing a belt, ゆるい may sound a bit more natural in some situations.
Still, 大きい is not wrong.

No, normal Japanese is usually written without spaces.

So the standard written form would be:

ベルトをしないと、このズボンは少し大きいです。

Spaces are often added in teaching materials to make the structure easier for learners to see.

So the version with spaces is helpful for study, but not how Japanese is normally written in everyday text.