What people look like -- part 2

I started out wondering what different authors say about how people look, but found myself moving on to wondering why. Why does Eliot spend so much space on Mr Cadwallader's looks, none at all on Sir James? Maybe it's because this is all we really need to know about Mr. Cadwallader. He's a flat character, in E. M. Forster's phrase.  He's keenly observed and memorable, and he's in the novel to do a job.

I don't know about Young Ladislaw. I'm only about 1/3 of the way through the book, and I assume he'll come back in and play a larger role. So why does he get essentially a Young Tad treatment, fleshed out with the prominent, threatening aspect to mouth and chin? I'm guessing it's because Dorothea would like to pigeonhole him, but she can't quite do it. 

Mr. Casaubon gets two vivid synechdocal descriptions, the deep eye sockets and the moles with hairs sticking out, because that's all that's needed. We have both views of him as well as could possibly be desired: we don't need to know what shape his nose his, or which direction his hair falls.