I've found that posts here go "wide" only when there's something -- a long text string, a picture -- "propping" the post wider than normal.

I've noticed that the software inserts a space into long text strings after a certain number of characters...
12345678101234567820123456783012345678401234567850123456786012345678701234567880
...apparently depending on the font size in use. That would ordinarily serve to keep the string from getting long enough to affect the post width. As I've mentioned before, this can even cause a long string of smilies (like the dancing chili) to get an unwanted break in the middle:


Pictures sometimes, not always, get scaled down automatically -- or maybe they all would if I were to stick around long enough to let them finish loading.

I don't have problems with signatures because I'm using the option that turns them off.