The Twitter sizing issue is exclusively a Twitter issue--as in the only people who can fix the issue is Twitter. Everything here is functioning as intended.
Which means, if I had to guess, the issue with the white background not appearing is Twitter scripting as well.
It also might be exclusively an iPhone issue (still because of Twitter scripting), because when I inspect mobile on my computer, tweets appear properly and do not extend past the max width of the page.
We're just going to have to wait until Twitter gets off their collective ass and does something to fix it.