OMG, she's almost identical to my cat Petunia! Check this out:
In answer to your question, some cats are just more vocal than others. If you're concerned about it I'd get her checked, but unspayed females tend to have lots more to say than spayed ones, even when they aren't in heat. Also, you may want to get her fixed soon; both my kittens started peeing in the house around 6 months old because they weren't spayed yet. This problem went away once they were fixed. I think your kitty looks about right for 6 months old, but size isn't always the best indicator. Petunia still looks like a kitten and she's 2.