Perhaps mine came from a strain of model bluebottles lol or perhaps my supplier kept his in the fridge to long so they lost weight.
I don't think the Maggots in general are as big as the ones one could get in say the 70's.
The really big ones were called gozzers (Bluebottle Larvae)
Maybe the Bluebottles used these days aren't as big & as well fed as they were due to commercial issues.
Some Tackle shops used to breed their own back in the day & not use commercial maggot farm sources.as is the order of the day - today!!
I got some huge ones off a pig skull one time and a nasty septic cut for my efforts