I'm going to disagree. 121 gm at 5 weeks is small, I don't care if it's hand fed or nursing from mom. I have hand fed many, many kits, and it seems on average they get larger than the kits that nurse from mom. How could they not? I stand there, every 2 hours, and feed them formula until they are stuffed enough that their tails curl over their backs and they totter away dragging their bellies. I don't agree with the comment "for hand fed babies" as being a reason why those kits are small. Ask anybody on forum who has seen Dixie or her brother. They were both hand fed and Dixie weaned at 350 gm at 8 weeks and her brother was 345 and they are both now in the 950 gm range as adults. But I fed them, around the clock, every 2 hours until they were 6 weeks old, then backed off to every 4 hours until they weaned.
IMO, your kits are small. How often are you feeding them, how much are they taking in, how much cereal do you add to the formula, and do they fight the feedings?