Currently, public sub counts are abbreviated in most but not all places across YouTube. In August, we’ll make this more consistent by always showing abbreviated sub counts publicly. Creators: You’ll still see your full sub count in Studio! Learn more → http://yt.be/help/abbrev-count …pic.twitter.com/pmaKk4EU4k
Just think of the bandwidth and money @TeamYouTube will save not sending those extra bytes! Besides, without a "M" and truncating the number, how will people know how big it is? It isn't like they can count that high anyway.