This question was a lecture discussion topic during my Biophysics course at university. I missed the lecture and the question is bothering me. "Eukaryotes (multi-cell organisms) evolved into very large sizes whereas prokaryotes (single-cell organisms) remained quite small (about 1 micrometer). What has prevented prokaryotes from growing to larger sizes?" I think the answer may be grounded in statistical mechanics but since I don't know the answer, that could be rubbish. Any help is appreciated!