По сравнению с пончиком этот кекс не такой сладкий, зато чизкейк кажется легче.

По сравнению с is a fixed expression meaning in comparison with or compared with.

After с in this expression, Russian uses the instrumental case, so:

• пончик = donut
• с пончиком = with / compared with a donut
• по сравнению с пончиком = compared with a donut

That is why you see пончиком, not пончик.

Because кекс is a masculine singular noun, and the demonstrative этот must agree with it.

Agreement here is:

• этот = this (masculine)
• эта = this (feminine)
• это = this (neuter)
• эти = these (plural)

So:

• этот кекс = this cake / this muffin-like cake

By contrast, это on its own often means this is or it is, as in:

• Это кекс. = It is a cake.

So этот кекс is correct because этот is modifying the noun.

Here it means not as sweet.

The pattern is:

• такой + adjective = so / such / as ... as
• не такой сладкий = not so sweet / not as sweet

In full, Russian often says:

• не такой сладкий, как пончик = not as sweet as a donut

But in your sentence, the comparison is already clear from по сравнению с пончиком, so как пончик is omitted.

So the idea is not that the cake is completely unsweet; it is just less sweet in comparison.

Yes, you could, but the nuance is a little different.

• не такой сладкий = not as sweet
• менее сладкий = less sweet

Both work, but:

• не такой сладкий sounds a bit more natural in everyday comparison
• менее сладкий can sound slightly more formal or analytical

So:

• Этот кекс не такой сладкий. = This cake isn’t as sweet.
• Этот кекс менее сладкий. = This cake is less sweet.

Both are correct, but the original wording is very natural conversational Russian.

Зато is a very common Russian contrast word. It means something like:

• but
• however
• on the other hand
• but at least
• in return / as a compensating good point

In this sentence, it introduces a positive contrast:

• the cake is not as sweet
• but the cheesecake seems lighter

So зато is not just plain contrast like но. It often suggests: there is a drawback, but there is also an advantage.

That is why зато fits especially well here.

Many borrowed nouns in Russian are assigned a grammatical gender, and чизкейк is normally treated as masculine.

You can see that because:

• it behaves like a typical masculine noun
• it ends in a consonant
• Russian says things like этот чизкейк

This is very common with loanwords that end in a consonant:

• сэндвич
• бургер
• маффин
• чизкейк

All of these are usually masculine.

Because легче is the comparative form of лёгкий:

• лёгкий = light
• легче = lighter

After кажется (seems), Russian can use:

1. a normal adjective/noun-like complement

   • чизкейк кажется лёгким = the cheesecake seems light

2. a comparative

   • чизкейк кажется легче = the cheesecake seems lighter

So the difference is:

• лёгким = light
• легче = lighter

If the meaning is comparative, you need легче.

Russian often leaves the comparison implicit if the context already makes it clear.

So кажется легче means:

• seems lighter

and the listener understands lighter than the donut or possibly lighter than the cake, depending on context.

If you wanted to make it explicit, you could say:

• чизкейк кажется легче, чем пончик
• чизкейк кажется легче, чем этот кекс

Russian does this a lot with comparatives: the чем + noun part can be omitted when it is obvious.

Because зато is a coordinating conjunction linking two clauses, and Russian punctuation normally puts a comma before it.

Here the two parts are:

• этот кекс не такой сладкий
• зато чизкейк кажется легче

So the comma marks the contrast between them.

This is similar to how Russian often uses commas before conjunctions like:

• но
• а
• зато

Russian word order is fairly flexible, but different orders change the focus or emphasis.

The original order:

• По сравнению с пончиком этот кекс не такой сладкий, зато чизкейк кажется легче.

puts the comparison frame up front: compared with the donut...

You could also say:

• Этот кекс по сравнению с пончиком не такой сладкий, зато чизкейк кажется легче.

This is still correct, but it sounds slightly different in emphasis.

So the original order is natural because it introduces the comparison immediately, then gives the two judgments: first about the cake, then about the cheesecake.