My baby is 5 months and weights about 18 pounds and is in size 9 month clothes , is that a normal weight??

My sister in law has a baby as well who is 6 months old and is about 20 pounds and in “18 months clothes”... I say this because she makes her baby wear big clothes and lose clothes because she says “if the baby wears tight clothes he or she might stay small..” like what???! How?? She’s always bragging how her baby was in 12 month clothes by the time her baby turned 5 months...

Just want to see what everyone else thinks. TIA