Nem mintha kicsi lenne a ruhaállvány, csak sok ruhadarab van rajta.

It is a common Hungarian pattern for rejecting one explanation and giving the real one.

Here it means something like:

• Not as if ...; it’s just that ...
• more naturally in English: It’s not that ..., it’s just that ...

So the speaker is saying:

• the problem is not that the clothes rack is small
• the real reason is simply that there are a lot of clothes on it

It often sounds softer and more natural than a blunt correction.

Because mintha usually goes with the conditional form of the verb.

So:

• mintha kicsi van would be wrong
• mintha kicsi lenne is correct

Here lenne is the conditional form of lenni / van, and in English it often corresponds to were in sentences like as if it were small.

So Nem mintha kicsi lenne is literally close to Not as if it were small.

Because Hungarian uses these two forms differently:

• kis is usually used directly before a noun:
  kis ruhaállvány = a small clothes rack
• kicsi is usually used when the adjective stands as the predicate:
  A ruhaállvány kicsi = The clothes rack is small

In this sentence, the meaning is the rack is small, not the small rack, so kicsi is the natural form.

Hungarian word order is flexible, and it often depends on what is being presented as the main descriptive part of the clause.

In this kind of sentence with lenni, it is very natural for the descriptive part to come first:

• kicsi lenne a ruhaállvány

This is especially common after mintha.

If you say:

• a ruhaállvány kicsi lenne

that is possible, but it sounds more marked and can give more prominence to a ruhaállvány.

So the original order is a very natural, neutral way to say it.

Ruhaállvány is a compound noun:

• ruha = clothes, clothing
• állvány = stand, rack, frame

So ruhaállvány means clothes rack, garment rack, or sometimes clothes stand, depending on context.

Hungarian makes compound nouns very freely, so words like this are extremely common.

Ruhadarab means an item of clothing or a piece of clothing.

It is built from:

• ruha = clothing, clothes
• darab = piece, item

So:

• sok ruha = a lot of clothes
• sok ruhadarab = a lot of clothing items / many individual garments

Using ruhadarab emphasizes the clothes as separate items. That fits well with a rack full of things hanging on it.

Because after numbers and quantity words like sok, Hungarian normally uses the noun in the singular form.

So Hungarian says:

• sok könyv = many books
• sok ember = many people
• sok ruhadarab = many clothing items

Even though the meaning is plural in English, Hungarian does not usually add a plural ending here.

So sok ruhadarabok would be incorrect in standard Hungarian.

Because sok ruhadarab is already an indefinite quantity phrase.

Hungarian usually does not add an article in front of this kind of expression:

• van ott könyv = there is a book there / there are books there
• sok ruhadarab van rajta = there are many clothing items on it

If you added an article, it would usually change the structure or sound unnatural here.

So the no-article form is the normal one.

Rajta means on it, on him, or on her, depending on context.

Here it means on it, referring back to a ruhaállvány.

So:

• van rajta = is on it / are on it

Hungarian does not say this with a separate word like English on + it. Instead, it uses one combined form:

• rajtam = on me
• rajtad = on you
• rajta = on him/her/it
• rajtuk = on them

In this sentence, rajta is very natural because clothes are hanging on the rack.

Because csak here means just or it’s just that, and that fits the structure.

Hanem is usually used when you directly correct a negated statement:

• Nem kicsi, hanem nagy.
  It isn’t small, but big.

But this sentence is not built that way. The first part is the idiomatic expression Nem mintha ..., not a simple negation.

So the natural continuation is:

• csak sok ruhadarab van rajta = it’s just that there are a lot of clothes on it

Using hanem here would sound unnatural.

Yes. You could also say:

• Nem mintha kicsi volna a ruhaállvány ...

Both lenne and volna can be used as conditional forms here.

Very roughly:

• lenne is extremely common and neutral in everyday speech
• volna can sound a bit more literary, soft, or stylistically elevated

In this sentence, lenne is completely normal and natural.