I'm thinking the bias edges are getting stretched as you sew. You could try pre-starching the fabric before you cut; heavy starching will keep the fabric more stable. I haven't tried starch with curves, though, so not entirely sure how this would work.
Sharon Schamber has videos on Youtube showing how to "true" blocks to size -- including blocks that are lopsided, too big, or too small. Your green block doesn't look too much off, so that is certainly an option. Here are the Youtube videos (some are continued to another video):
http://www.youtube.com/watch?v=v-3RIWhBvcA
http://www.youtube.com/watch?v=i6aplw_tVZc
Mary