I'm sure you know you can't discriminate against them because of their size. Have the original tenants broken the lease or rental agreement? Most leases don't allow additional tenants to just move in without the approval of the landlord and would require a new or amended lease.
You can tell them the proper way to use the fridge in writing, but be careful that you don't sound like you're saying they eat too much. How much they eat isn't the landlord's business, and again, you don't want to discriminate against them because of their weight.
If you do an inspection with appropriate advance notice, you have a right to see every room. So if you didn't see the bedrooms last time, I suggest you do another inspection. If the rental or lease agreement has been violated, take the steps outlined in the agreement to correct it. For most issues, my leases give the tenant 7 days to make it right. If they don't correct the issue in time, they've broken the agreement and I can ask them to leave.
Good luck with this!