Have you brought them to a vet yet? They should be weighing your bunnies and telling you I’d they’re the correct weight.
I wouldn’t worry too much about the younger rabbits yet. They’re still growing a lot and sometimes the weight varies because some just grow a lot faster than others. One of my rabbits was significantly smaller than the other one when they were young, even though they were from the same litter. Now they’re both very close in weight.
Mini Lops should weigh around 5 pounds, a pound more or less should be ok. Lion Heads should be from 2 1/2- 3 3/4 pounds.
Here’s an article I found to help you determine if your rabbits are a healthy weight:
http://www.therabbithouse.com/diet/rabbit-weight.asp
The more important thing to know is if their diet is healthy and if they’re getting enough time outside of their pens to exercise.
Could you possibly explain what their diets are? What they’re eating and how much?
Also since you’re new, how are your bunnies housed? Are they all together or separated? What are their genders? And have they been altered?(spayed/neutered)