How bad is it for a rat to over eat?

So I've been switching my rats diets from a mix of lab blocks (oxbow) and seed mix (Reggie rat) to plain lab blocks (harland teklad). They also get a breakfast and dinner of fresh veggies, fruits meat etc. But what I'm concerned about is the lab blocks. Last night was the first night I filled up their tray with the new blocks and when I woke up this morning I noticed a substantial amount gone. About half the bowl! I looked around to see if they'd horded it anywhere but only found a few stashed away. Now, my two small females are laying around like they just ate a thanksgiving feast, round bellies and all! How bad is this for them? Should I be concerned? Is it normal for rats to do this when their diet is switched? Obviously I'll have to limit their lab blocks from now on until they learn not to be so greedy! It's strange, because I've heard of the exact opposite rats won't eat) happening quite often when switching to lab blocks quite often!