My boobs can't really be THAT big?

Casual

I just measured myself for the first time but I measured a 31D? My boobs aren't that big and I'm worried my mam won't believe me when I want to try a 31D bra! I really need some explanation because I'm in disbelief, and I spent ages getting an accurate measurement so im sure it isn't wrong! Does anyone have a credible explanation they can link to me?