Babies are actually very small, so you might be surprised!! I remember having store bought 'newborn' sized caps for my first son that looked so tiny, but his teeny little head was just swimming in them.
Does the bootie pattern you're using have finished measurements with it??
I think that using worsted instead of sport weight is a good way to make the bootie a bit bigger. That, and maybe going up a needle size from what is recommended.
(pdf file) is a lovely bootie pattern that I've always admired but have yet to try.