This has happened to me with all three of my kids. Days, weeks and months go by, and because I am with them every day, I still see them as teeny tiny -- and then one day, I will go up to check on them while they are sleeping, and bam! They look enormous.
Caroline is suddenly taking up the whole crib! When did THAT happen? My little baby is looking MIGHTY toddler-ish all of a sudden. She will be one on Saturday! Better bring some hankies for her mommy!