If women weren't elastic to some extend in that area they would break when giving birth. Yes, she can hold 7 inches - she can pop a freakin' baby when she has to!
So there are good odds you fill her out as well - the sensation is different than that of the dildo, but it doesn't mean she doesn't feel "filled up" with you or that you're not big enough.
Maybe you should read up on the female anatomy...
So chillax. There are many reasons why penises are just as nice or nicer than dildos.
1) They are attached to men. Can be both a pro and a con
2) They are flexible and follow the inner curves of the vagina, which stimulates certain areas that dildos don't/can't.
3) If the penis is attached to a guy then often that guy has hands and his own will. He can use said hands and maybe even his tongue in surprising ways according to before mentioned will. With a dildo there are only the dildo and no surprises... After awhile it gets boring.
4) Biology. The brain is clever and will sometimes not award the girl with all the nice hormones that make sex awesome if the girl doesn't have sex that could, perhaps, if the condom breaks and the pill stops working or whatever, lead to babies.
The reason why our bodies want us to have sex is to have babies and ensure the genes.
The orgasms are new and nice, yes, but can over time feel a little shallow and not as satisfying because certain hormones won't be released if the dick is not a real one.
So there you go.