大 can also be read as おお, as in 大盛/おおもり (eg at McDonalds: I'd like that supersized, please).
The reason 大 isn't used with 乳 is probably because its usage isn't always about literal/physical size but often used to "quantify" abstract things (in terms of varying degrees): think 大好き, 大ピンチ, 大雨...