The frustrating this is I've tried a size 9 (5.5mm), and when it buckled, I went to a size 11 (8mm), and it's still buckling. Sure, I can block it, but I'd kind of like to know why this is happening in the first place.

I don't want the regular part of the design (without the lacy holes) to be too open because I'm afraid it'll look sloppy. Is it possible that I really have to go larger?
On another note, the name of the stitch pattern is called "lace entrelac", although I don't think it's true entrelac (just knitting and purling across every row with the usual decreases and increases for the lace pattern)--would something that likens itself to entrelac buckle?