El que no entenc és per què aquesta talla és més cara que l'altra.

Literally, it means the thing that I don’t understand.

In more natural English, this is usually just what I don’t understand. Catalan often uses el que where English can use what by itself.

So:

• El que no entenc = what I don’t understand
• more literally = the thing that I don’t understand

This is a very common structure in Catalan.

Because the sentence is built like this:

• El que no entenc = the subject of the sentence
• és = is
• per què aquesta talla és més cara que l'altra = the complement explaining what that thing is

So the structure is exactly like English:

• What I don’t understand is why...

Without és, the sentence would feel incomplete.

This is one of the most important spelling/grammar distinctions in Catalan.

• per què = why / for what reason
• perquè = because

In your sentence, it is per què because it introduces an indirect question:

• No entenc per què... = I don’t understand why...

Compare:

• No entenc per què és tan car. = I don’t understand why it is so expensive.
• És tan car perquè és de marca. = It is so expensive because it is a brand name.

The accent helps distinguish què from que.

• què is usually an interrogative form: what? / why? in expressions like per què
• que is usually that, which, or a relative/conjunction form

Here, per què is asking for a reason, so què is stressed and takes an accent.

That means:

• per què = why
• perquè = because
• que = that/which

Because this is an indirect question, not a direct one.

In indirect questions, Catalan normally keeps the usual statement word order:

• per què aquesta talla és més cara...

This is similar to English:

• direct question: Why is this size more expensive?
• indirect question: I don’t understand why this size is more expensive.

Notice that English also stops using inversion in the indirect question. Catalan works similarly here.

Here, talla means size, especially a clothing or shoe size.

So in this context, aquesta talla means:

• this size

Be careful: talla can have other meanings in other contexts, but when talking about clothes, it commonly means size.

For more general physical size or dimensions, Catalan often uses mida instead.

Because talla is a feminine singular noun, and the demonstrative has to agree with it.

So:

• aquest = this, masculine singular
• aquesta = this, feminine singular

Since talla is feminine:

• aquesta talla = this size

Agreement is very important in Catalan for articles, demonstratives, and adjectives.

Because cara must agree with talla, which is feminine.

The adjective is car / cara = expensive

So:

• un jersei car = an expensive sweater
• una talla cara = an expensive size

In your sentence:

• aquesta talla és més cara = this size is more expensive

Even though in English expensive never changes form, in Catalan the adjective does.

Més ... que is the normal Catalan pattern for more ... than.

So:

• més cara que l'altra = more expensive than the other one

The pattern is:

• més + adjective/adverb + que

Examples:

• més gran que = bigger than
• més ràpid que = faster than
• més cara que = more expensive than

So que here means than, not that.

Because la becomes l' before a vowel.

So:

• la altra is not used
• l'altra is the correct form

This is called elision. Catalan does this very often with articles:

• l'amiga
• l'escola
• l'altra

Since altra begins with a vowel, la changes to l'.

Because Catalan, like English, often leaves out a repeated noun when it is obvious from context.

So:

• l'altra really means l'altra talla

But saying just l'altra is more natural because the noun has already been mentioned.

This is similar to English:

• this size is more expensive than the other one

Catalan often uses the article + adjective in this way when the noun is understood.

Yes. That is perfectly natural.

The difference is mainly one of emphasis:

• No entenc per què aquesta talla és més cara que l'altra.
  = I don’t understand why this size is more expensive than the other one.

• El que no entenc és per què aquesta talla és més cara que l'altra.
  = What I don’t understand is why this size is more expensive than the other one.

The version with El que no entenc és... gives extra focus, as if the speaker is highlighting the confusing part.