Комиссия оказалась меньше, чем я ожидала.

Here комиссия most likely means a fee or commission charge, for example a bank fee, service fee, or transaction commission.

It is a feminine singular noun, which matters for the verb forms in the sentence.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject here is комиссия, which is feminine singular, so the verb is оказалась.

Compare:

• платёж оказался меньше — payment is masculine
• комиссия оказалась меньше — commission is feminine
• расходы оказались меньше — expenses is plural

Оказалась means something like turned out to be or ended up being.

So:

• Комиссия была меньше = The commission was smaller
• Комиссия оказалась меньше = The commission turned out to be smaller

Using оказалась adds the idea of discovery, result, or a contrast with what someone expected.

Меньше is the normal comparative form meaning smaller / less.

In Russian, forms like меньше, больше, лучше, хуже are usually invariable in this kind of sentence. They do not change for gender, number, or case.

So even though комиссия is feminine, you still say:

• комиссия меньше
• сумма меньше
• платёж меньше

Меньшая is a different adjective form and would not be the natural choice here.

Here чем means than.

The pattern is:

• меньше, чем... = smaller/less than...
• больше, чем... = more/bigger than...

So меньше, чем я ожидала means smaller than I expected.

Because чем я ожидала is a subordinate clause with its own verb, ожидала.

Russian normally puts a comma before a subordinate clause:

• Комиссия оказалась меньше, чем я ожидала

The comma is required here.

Yes. In the past tense, Russian often shows the speaker's gender when the subject is я.

• я ожидал — a male speaker
• я ожидала — a female speaker

So ожидала tells you the speaker is female.

Because Russian often leaves out an object if it is already clear from context.

Here чем я ожидала means than I expected. The sentence does not need to repeat exactly what was expected, because it is understood from the previous part: we are talking about the size or amount of the commission.

English can do the same thing:

• It was smaller than I expected

You do not have to say than I expected it to be every time.

Yes. Ожидать is an imperfective verb, and that fits well here because it describes a prior mental expectation, not one completed action with a clear endpoint.

Russian commonly uses ожидать in this kind of sentence:

• Я ожидала большего
• Это было лучше, чем я ожидала

So ожидала here is the normal and natural choice.

Yes, but Комиссия оказалась меньше, чем я ожидала is the most neutral and natural order.

Russian word order is flexible, but changing it usually changes emphasis, not the basic meaning.

For example:

• Комиссия оказалась меньше, чем я ожидала — neutral
• Оказалось, что комиссия меньше, чем я ожидала — slightly more focus on the fact that this was discovered
• Меньше, чем я ожидала, оказалась комиссия — unusual, strongly marked emphasis

So the original sentence is the best standard version for everyday use.