One of the things that stops bacteria from being super big is surface area. Most things in biology are down to surface area. Unlike cells of so-called eukaryotes, they lack a lot of the specialized infrastructure used to get stuff where it needs to go (although they have a sort of cytoskeleton analogue). Still, for bacteria, they largely need to rely on sheer probability to get stuff where it needs to be. It's a gamble. Stuff moves around randomly but if you're small enough, they'll slot into where they belong most of the time. Bacteria are chaotically-aligned beings, you understand.

This is still one of my favorite paragraphs. "Most things in biology are down to surface area", I like that a lot