40s Dress Muslin Crashes and Burns!

I swear I had the right size! I was wearing the correct bra! But boy howdy, this does NOT fit! I'll try again. I like the way it fits, even if not the actual 'fit' itself. I'm going to have to go back to the thrift store for more sheets though to make another muslin. I added in seam allowances but this is a huge size difference. Look at how it strains across the back --I could hardly get my arms in it. There is absolutely no way that it would meet across the front. Tight all over. It does seem as though it will be a flattering suit though --once I get the size right.