It's a given that the aquatic form (just before emergence) of bugs will be smaller than the emerged specimen, at least from the standpoint of mass. In general, the mayflies, chironomids, caddis and stonies don't eat once they come out, and then there's the exoskeleton shuck they left behind. To me a size 16 emerger will translate into an 18 once emerged.
But the cranes, at least locally here in Chester County, are the most astounding for size disparity. The VFTU runs a fly fishing school and we often collect a mess of bugs and put them in a dishpan for the students to observe. Cranefly larvae are sometimes 60mm or more in those collections.
One of the members was hatching various bugs in an aquarium, and saw that those giant larvae hatched into size 18s.
tl
les