The shorter the width of a text block, the less options the system has to craft a justified text with consistent word spacing. So it works better with larger blocks of text.
This said, one solution could be to just ditch justifying. Justifying is imho a thing from the past and/or reserved to who has a lot of control on the text. Justifying is usually reserved for when you can manually address all exceptions and weirdness, either in a print shop with lead fonts, or in Indesign where you control widows and orphans, among many other things.
On the web, justifying looks bad, most of the time. Also, it’s not more legible for users, it’s less legible. So it’s really a cosmetic thing. If your client want it, you can shop for counter arguments on Google, there are plenty. 1st reason is that by the third line there are already weirdnesses, and you can do little if nothing against it. Check the paper versions of New Yorker or New York time, their justified text is flawless, and it demanded a lot of attention and the use of many features that CSS don’t have.