I packed quite a bit of stuff in my Sunlite saddlebags, but they're not waterproof, nor are they very rigid. Thankfully, none of my clothes needed to be ironed.

I would pack a pair of jeans/pants in a large zip-lok bag, and squeeze the air out of it to make it as flat as possible. I did the same with shirts, undies, etc. in another bag, and squished them flat. I could easily fit a full change of clothes in 1 pannier this way, plus it was waterproof, and made it easier to find things.

In the other pannier, I could fit lunch fixings, extra shoes, towels, etc.