Blades don't have to be thick, and transmutations are a simple exchange from one form to another on a 1:1 ratio with the same mass. It's entirely possible he's just reducing the thickness of the outer plates to make the blade, something that wouldn't be visually apparent. Ed and Al do, at least, know about producing more metal by thinning a plate, because in the episode of Brotherhood where Ed and Al meet Father, he replaces Al's broken arm, and they both comment that he did so WITHOUT thinning his armor, which they then conclude must mean he has a Philosopher's Stone.