Rainbow dash does not strike me as a big chested girl mostly as the athletic types tend not to be stacked in RL; fliers wings also burns up a lot of the excess energy they might have so no fat means smaller breast size so doubly doubtful of that.
If you going to make her like if she was human then she'd be a modest chest with well defined legs and butt as she is a runner so that is going to toned as hell. Most fanart that's not porn has them as normal sized girls.
Offtopic: breast size smallest to largest: Pinkie Pie, A; Rainbow Dash, B; Twilight Sparkle, C; Applejack, D; Rarity, D; Fluttershy DD. Rarity knows she has a good figure and AJ is a country girl so bigger breasts are expected.
Over a decade of Anime-Expo(00,'04-'15)
Cons this year ('15).
Phoenix Comic Con -One day only.
Anime Expo - Next
"Kilts were of course everyday wear. In battle we'd don full length sequin gowns to blind our enemies." -Groundskeeper Willie.