Here's a quick little size comparison between captive female and wild caught female Chinese Mantis.  The captive raised from hatch and the wild caught was found here in Omaha NE.  The wild caught is about 25% bigger than the captive.  This is known thing in the mantis community.  Similarly fertility is also markedly higher in wild caught specimens.  The assumption is that is has to do with nutrition, particularly the lack of pollinators as a food source.  You can actually buy pollen to coat or gut load your feeders with.  Supposedly it increases size and fertility but I haven't messed with it enough to have an opinion.