MIL & clothes

So my mother in law has gotten us a lot of clothes for the baby (this is her first grandchild, so I expected it) anyway for some odd reason she thinks that however old the baby is your supposed to double it and that’s what size clothes she will be in. For example, she thinks when the baby is 3 months old she will be wearing 6 month clothes. She got her this really cute one piece outfit thing but it’s pants and long sleeves and it’s 6-9 months, at that age it will be right in the dead of summer, it will be to hot for her to wear it 🤦‍♀️ I don’t know where she heard this from but it’s very strange to me. So while I love and appreciate all the clothes we have gotten from her I’m worried that the baby won’t be able to wear many of them because they are gonna be off as far as seasons goes. Has anyone ever heard of this before? It’s kinda frustrating, I don’t want these clothes to go to waste or her be upset because the baby didn’t wear any.