Understandably, the more a tab loads, such as a heavy page full of large ads, the larger that tab's helper process will be. Conversely, if one loads a basic HTML file with just a few paragraphs of text, that tab will be ~20MB.
But then, why does the browser process itself take up 400-500MB, and for that matter the GPU renderer another 500-800MB? What leads to this bloat?