I don't know how exactly to measure breast cancer risk, it ofc would make sense that bigger volume of tissue = more places for growth becoming cancerous. I'm quite certain though that environmental and genetic factors are far greater than breast volume.
If you want them big fast, then just go for implants instead. NBE, even when done on top of conventional HRT (which btw, is the only very certain way to breast development.) is not a sprint, but a marathon. It takes years no matter what you do. Typical breast development takes five to ten years to complete and some times even longer as growth potential exists for life.
You will not see massive changes with just estrogen creams. If you want it all, you have to do it all and that means combining conventional HRT with all the best NBE methods and even then its a roll of dice with genetics, age and luck. Don't fool yourself into thinking this is fast or easy or that there's any other guaranteed way to big boobs than implants. Everything else is slow and gradual and there's ton of factors to it.
its the best to consider your methods and needs quite a bit before you commit yourself to this anyway. I see too many men here being all fired uop by their boob fetish who think there's a magic pill you can take to get to it overnight. That's not how this stuff works.
I suggest you do more reading on the forum, there's tons of great advice, Project-X thread by Lotus is a massive read of thousands of posts, there's a lot of good information in there but it can be a chore to dig through, well worth it though. Then there's Topical Cream Protocol thread again by Lotus and myself as I've done quite a bit of tweaking and experimenting on the said program. Then Freya/Valkyrie has posted interesting stuff about massage techniques, Squirrels threads are worth checking out and I guess my main thread might be the easiest read for having an overview about what typically works for many people here.
Just remember that no matter what you do, you can't rush breast growth. Its more about resielience of watching paint dry, it takes years regardless of methods, excluding plastic surgery.