It really depends on the pattern, the yarn, the gauge, and how much it felted. The booga bags I've done have been sorta floppy, but not overly so (I did them both in Noro Kureyon). Other bags I've done (like the one on the cover of this book
in Cascade 220) have been quite rigid.
If you don't like it, though, lining in a fairly stiff fabric can help. Fusible interfacing might also help.