deetayouryou ga ookii.

が marks データ容量 as the grammatical subject of 大きい.

So the structure is basically:

• データ容量 = data capacity / storage size
• が = subject marker
• 大きい = is large

This is a very normal pattern in Japanese:
[noun] が [i-adjective]

For example:

• 音が大きい。 = The sound is loud.
• 部屋が広い。 = The room is spacious.
• データ容量が大きい。 = The data capacity / data size is large.

If you used は instead, the nuance would shift more toward topic or contrast:

• データ容量は大きい。

This can sound like:

• “As for the data capacity, it’s large.”
• “The data capacity is large, though maybe something else is not.”

So が is the more neutral choice if you are simply stating that the data capacity is large.

データ容量 is a compound noun:

• データ = data
• 容量 = capacity, volume, storage size

Together, データ容量 usually means something like:

• data size
• storage capacity
• amount of data
• file size / memory size, depending on context

The exact English translation depends on what is being discussed. For example:

• for a file: file size
• for a device: storage capacity
• for a plan or transfer limit: data allowance / data capacity

So it is a broad technical phrase, and context tells you the best English wording.

Yes, 大きい literally means big / large, but in Japanese it is used very naturally for things like:

• size
• scale
• capacity
• volume

So 容量が大きい is a standard way to say:

• the capacity is large
• it has a lot of capacity
• the data size is large

This does not mean the object is physically big. It refers to the amount/capacity.

Japanese often uses big/small where English might use more specific words:

• 容量が大きい = large capacity
• 音が大きい = loud
• 影響が大きい = significant / major impact

So this is a good example of how Japanese 大きい covers a wider range than English big.

Both are possible, but they feel a little different.

データ容量

This is a compound noun. It is compact and common in technical, written, or formal contexts.

データの容量

This is more explicitly:

• “the capacity of the data”
• “the data’s capacity/size”

This version can feel a bit more explanatory or less technical.

In many technical contexts, Japanese prefers the compact compound form:

• 通信速度 = communication speed
• 保存容量 = storage capacity
• データ容量 = data capacity / data size

So データ容量が大きい。 sounds perfectly natural.

Yes. データ容量が大きい。 is a complete sentence in plain style.

Japanese has different levels of formality:

• Plain: データ容量が大きい。
• Polite: データ容量が大きいです。

Both are grammatical. The version without です is common in:

• casual speech
• notes
• technical writing
• headlines
• explanations in plain form

So the sentence is complete as written.

大きい is an い-adjective.

That matters because it can end the sentence by itself:

• データ容量が大きい。

Unlike a na-adjective or noun, it does not need だ to finish the sentence in plain form.

Compare:

• 大きい → can end a sentence on its own
• 静か → needs だ in plain form: 静かだ
• 学生 → needs だ in plain form: 学生だ

So here, 大きい is already doing the job of the predicate.

Yes. By itself, データ容量が大きい。 is context-dependent.

It could refer to:

• a file being large
• an app using a lot of storage
• a device having large storage capacity
• a mobile plan having a large data allowance
• a piece of data being heavy/large in size

Japanese often leaves the topic understood from context. For example:

• talking about a video file → “The file size is large.”
• talking about a smartphone → “It has a large storage capacity.”
• talking about a data plan → “The data allowance is large.”

So the grammar stays the same, but the best English translation changes depending on the situation.

Usually, 大きい is more natural than 多い with 容量.

Why?

• 容量 is a capacity/size concept
• 大きい is the natural adjective for something measured as large/small
• 多い is more natural for “many” or “a lot” of items/amounts

So:

• 容量が大きい = natural
• 容量が多い = unnatural or odd in most cases

Compare:

• データが多い。 = There is a lot of data.
• データ容量が大きい。 = The data size / storage capacity is large.

These are similar, but not the same.

Using は instead of が makes データ容量 the topic.

So:

• データ容量が大きい。 = The data capacity is large.
  (neutral statement)

• データ容量は大きい。 = As for the data capacity, it’s large.
  (topic/contrast nuance)

The は version may imply comparison or contrast, such as:

• データ容量は大きいが、処理速度は遅い。
  “The storage capacity is large, but the processing speed is slow.”

So が is often better for a simple observation, while は is useful when setting up a topic or contrast.

Yes, very much so.

In English, we usually choose different adjectives depending on the noun:

• a big box
• a loud sound
• a large capacity
• a major influence

In Japanese, 大きい can cover many of these ideas:

• 箱が大きい = the box is big
• 音が大きい = the sound is loud
• 容量が大きい = the capacity is large
• 影響が大きい = the impact is great

So one common learning point is that Japanese often uses 大きい more broadly than English uses big.

The basic order is very natural:

• データ容量 が 大きい。

Japanese usually puts the predicate at the end, so this is the expected order.

You generally would not move 大きい before データ容量 in a normal sentence. However, you can expand the sentence in front of the noun or around it:

• この機種はデータ容量が大きい。
• このファイルはデータ容量が大きい。

So the sentence-final adjective pattern is standard, and that is what you are seeing here.