No, not all Italian women have big boobs, but you must first define what you mean by big boobs.
Larger than size 36, American ratings? What cup size, D at a minimum?
If you would like a scientific answer you need to define your terms first.
Italian cinema has probably given birth to this notion, what with the cinematic fantasies of Fellini and the wonders of Sophia Loren, Gina Lollibrigida, etc.
Generally speaking Italian women probably have larger breasts than other European women, and it is probably possible to find statistics to support this thesis. Again, however, please define your terms.