По сравнению с круглым столом квадратный занимает меньше места, но за ним помещается меньше людей.

По сравнению с means compared with or in comparison with.

It is a fixed expression, and the noun after it goes in the instrumental case:

• по сравнению с круглым столом = compared with a round table
• по сравнению с прошлым годом = compared with last year

So in this sentence, it sets up the comparison before the main statement.

Because по сравнению с requires the instrumental case.

The basic dictionary form is:

• круглый стол = a round table

In the instrumental singular, that becomes:

• круглым столом

Both words change because the adjective must agree with the noun in gender, number, and case:

• круглый → круглым
• стол → столом

Russian often leaves out a noun if it is obvious from context.

Here, квадратный literally means square, but it really means квадратный стол = the square table / a square one.

English does this too:

• The round table is big, but the square one is smaller.

So in Russian:

• круглый стол = round table
• квадратный = the square one

This is very natural.

After words like меньше and больше, Russian usually uses the genitive case.

So:

• место = space, place
• меньше места = less space

This is a very common pattern:

• меньше времени = less time
• больше денег = more money
• меньше работы = less work

Also, занимать место means to take up space or to occupy space, so:

• занимает меньше места = takes up less space

Yes, literally за ним can mean behind it, but with tables, Russian often uses за where English uses at or around.

For example:

• сидеть за столом = to sit at the table
• собраться за столом = to gather around the table

So:

• за ним помещается меньше людей means fewer people fit at/around it

Here ним refers back to стол.

Because за here expresses location, it takes the instrumental case, so:

• он → ним

This is a very common Russian pattern with quantity words like:

• много = many/a lot
• мало = few/little
• больше = more
• меньше = fewer/less

After these words, the noun is usually not the grammatical subject in the normal nominative plural sense. Instead, Russian treats the whole quantity expression as a single idea.

So:

• помещается меньше людей = fewer people fit

The verb is therefore commonly singular.

You can compare:

• Здесь работает много людей. = Many people work here.
• На площади собралось много людей. = A lot of people gathered in the square.

In the second example, the verb is also singular.

Because after меньше, Russian uses the genitive plural.

The basic form is:

• люди = people

But after a quantity expression, it becomes:

• меньше людей = fewer people
• много людей = many people
• несколько людей = several people

So людей is the correct form here.

Помещаться means to fit, to have room, or to be able to be accommodated.

In this sentence:

• за ним помещается меньше людей

it means:

• fewer people can fit around/at it

It is related to the idea of space and capacity.

Compare:

• В коробке помещается десять книг. = Ten books fit in the box.
• В машине помещается пять человек. = Five people fit in the car.

So here it is about how many people the table can accommodate.

The word order here is natural and clear, but Russian word order is more flexible than English.

This version:

• По сравнению с круглым столом квадратный занимает меньше места, но за ним помещается меньше людей.

sounds neutral and well-balanced. It first gives the comparison, then states two contrasting facts:

1. the square one takes up less space
2. fewer people fit around it

You could move words around for emphasis, but the original is a very natural way to say it.

For example, Russian often puts important or contrasting information later in the clause, which is part of why меньше места and меньше людей come where they do.