Learn Hardcore French: Paul a trop mangé de pizza, et maintenant son ventre est lourd. - Paul ate too much pizza, and now his stomach feels heavy.

In compound tenses like the passé composé, short adverbs (such as trop, bien, mal, beaucoup, peu) normally go between the auxiliary verb and the past participle:

Paul a trop mangé.
Il a bien travaillé.
Elle a beaucoup parlé.

So a trop mangé is the most natural word order.

a mangé trop is not wrong, but it is less common and feels a bit heavier or more marked. It can sound like you’re insisting on the “too much” at the end, but in everyday speech people would almost always say a trop mangé here.

Is there a difference between a trop mangé de pizza and a mangé trop de pizza?

Both are grammatically correct, but the nuance is slightly different:

Paul a trop mangé de pizza.
Focus: he ate too much (and what he ate too much of happens to be pizza). The “too much” applies to the action of eating.
Paul a mangé trop de pizza.
Focus: he ate too much pizza in terms of quantity. The “too much” directly modifies the amount of pizza: trop de pizza.

In everyday speech, both can be used to mean essentially the same thing, but a trop mangé de pizza is a bit more idiomatic in this exact sentence.

Why is it de pizza and not de la pizza or ?