الكم واسع، في حين أن الكفة ضيقة.

A fully vowelled version is:

الكُمُّ واسعٌ، في حينِ أنَّ الكُفَّةَ ضيِّقةٌ.

A helpful transliteration is:

al-kummu wāsiʿun, fī ḥīni ʾanna al-kuffata ḍayyiqatun.

In normal speech, especially when pausing, the final case endings are often not pronounced, so it may sound more like:

al-kumm wāsiʿ, fī ḥīn ʾanna al-kuffa ḍayyiqa.

Because in Arabic, the present tense of to be is usually omitted.

So:

• الكم واسع literally looks like the sleeve wide
• but it means the sleeve is wide

This is a very common Arabic pattern, called a nominal sentence.

If you wanted was, you would normally use كان:

• كان الكم واسعًا = the sleeve was wide

Here, الكم means the sleeve.

A useful note: the spelling كم can also be related to other meanings in Arabic, such as how much/how many or quantity, depending on context. But in this sentence, because of the clothing context, الكم clearly means the sleeve.

Here الكفة most likely means the cuff or the edge/opening of the sleeve.

In clothing vocabulary, كفة can refer to a cuff, hem, or edge, depending on context. Since the sentence compares it with the sleeve, cuff is the most natural understanding here.

Because the adjective must agree with the noun in gender.

• الكم is masculine, so the adjective is واسع
• الكفة is feminine, so the adjective is ضيقة

So:

• واسع = masculine singular wide
• ضيقة = feminine singular narrow

The feminine form often adds ة:

• واسع → واسعة
• ضيق / ضيّق → ضيقة / ضيّقة

Because واسع and ضيقة are not being used as ordinary descriptive adjectives inside a noun phrase. They are the predicate of the sentence.

Compare these two patterns:

• الكم واسع = the sleeve is wide
• الكم الواسع = the wide sleeve

In the first one, واسع is the predicate, so it is normally indefinite. In the second one, الواسع is an attributive adjective modifying الكم, so it must match it in definiteness.

This is a very important distinction in Arabic.

It is a basic nominal sentence:

• الكم = the subject
• واسع = the predicate

In traditional Arabic grammar, these are:

• مبتدأ = الكم
• خبر = واسع

So the structure is:

• [definite noun] + [predicate adjective]
• meaning [noun] is [adjective]

في حين أن is a connector meaning while, whereas, or while it is the case that.

In this sentence, it introduces a contrast:

• الكم واسع = the sleeve is wide
• في حين أن الكفة ضيقة = whereas the cuff is narrow

So the whole sentence sets up a comparison between two features.

It sounds a bit more formal than just using but.

In في حين أنَّ الكفة ضيقة, أنَّ introduces a full clause.

A very important grammar point in formal Arabic is that أنَّ affects case:

• its noun becomes accusative
• its predicate stays nominative

So in a fully vowelled version:

• أنَّ الكُفَّةَ ضيِّقةٌ

Here:

• الكفةَ is the noun of أنَّ
• ضيقةٌ is its predicate

In normal unvowelled writing, this case change is usually not visible, so learners often do not notice it at first.

Yes, often you could say:

الكم واسع، لكن الكفة ضيقة.

That would mean:

The sleeve is wide, but the cuff is narrow.

The difference is mainly one of style and nuance:

• لكن = simpler, more direct, like but
• في حين أن = more formal, more comparative, like whereas

Both are correct, but في حين أن sounds a bit more literary or formal.

The context of the vocabulary makes that clear.

• الكم = sleeve
• الكفة = cuff / edge
• واسع = wide
• ضيقة = narrow

These words naturally fit a description of a garment. Even if one word could have another meaning in a different context, the sentence as a whole points strongly to clothing.