Perhaps the confusion comes from the word “pan”. It has 2 meanings: bread (in general) and also “loaf” or “piece of bread”, as in “un pan de pueblo”, “un pan de hogaza” “un pan gallego”, etc, etc.

Eleena is, as usual, right: “comí pizza” means I ate some pizza (similar to the French partitive), “me comí una pizza” means I ate an entire pizza, “comí una pizza” sounds strange to me but would mean something like “I had a pizza for lunch”.

On the other hand, “me comí pizza” is simply not Spanish, doesn’t make any sense.

Por ejemplo, hay una diferencia entre “comí pizza” vs. “me comí una pizza.”

Persona 1: ¿Qué comiste ayer?
Persona 2: Comí pizza. (I ate pizza) [Could've been one slice, two slices, whatever, the person isn't saying. They're just stating that they ate pizza.]

Persona 1: ¿Qué comiste ayer?
Persona 2: Tenía tanta hambre que me comí una pizza. ¡Estaba riquísima! (I was so hungry that I ate an entire pizza. It was delicious!) [In this case, it may be an exaggeration that the person ate an entire pizza but what is clear is that the speaker wants to indicate that he ate a lot of pizza, more than what would be considered normal.]

No sé, me parece que si alguien tiene mucha hambre, la idea de “comerse un pan” sería posible (El hombre se comió un pan. The man ate an entire loaf of bread.), ¿pero a lo mejor tiene un matiz extraño en México con la palabra “pan”?

Actually, “aprovecharse de” means to take advantage in the sense of exploiting, but it does not have to be of a person.
Ex: Se han aprovechado de la situación.

“Aprovechar” is used in the sense of making the most of something, and it does not use the preposition “de.”
Ex: Quiero aprovechar esta oportunidad al máximo.

un saludo