First time for Everything

So, yesterday, for the first time, I bought size 5 diapers. Could my baby really be that big. I have only ever seen that size of a diaper on like 2 year olds....he is only 15 months. He just seems so big to me. I just can't imagine that as long as it took to potty train Ella, she was never any bigger than a size 4, and that was only at the end. But, my 15 month old baby is already a size 5. The thing that stinks about wearing bigger diapers is that there aren't as many in the package...so I feel like I get ripped off. Oh well, whatever holds it all in is worth it, I guess.