Cache strapless fancy dress

I bought this new at Cache and wore it one time for my bachelorette party. It's such a fun, fitted strapless dress, with light boning in the front and zipper in the back. The sizing is extremely weird and I think it's mis-sized. The tag says 10 but it fits like a 4/6 I think. Armpit to armpit about 16", but is formed at the bust so possibly a little bigger. Length from armpit is 25".