I'm not sure 'nowadays' is accurate
It's been bloatware for as long as I can remember, even going back to whatever version was supplied for Windows 3.1. It's been reported that when Apple were forced to graft Carbon onto OS X, as a transition technology from the classic OS, they found an incomplete but much cleaner implementation of the usual QuickDraw/etc stuff in the Windows port of QuickTime and worked forward from that. I appreciate that the thing was meant to do a lot more than just video but throwing large chunks of the system libraries for an OS in there sounds like it was the offence.
At a guess, the culprit is whoever decided that QuickTime needed to be a 'multimedia platform' rather than just a video playback tool. Comparisons with Apple's feelings about Flash are entirely appropriate.
They've fixed it on the Mac side as of QuickTime X, by the way — it's a clean break reimplementation thing that really just plays a subset of the video codecs that classic QuickTime had accumulated with none of the wider aspirations. I've no idea how they would defend what they currently ship for Windows but I doubt the defence would be very convincing.