I have the impression that D cells leak less as well but, as CatalinaWOW mentioned, it may be a numbers game. However, the D cells on my flashlights take much more abuse than the pampered AAs or AAAs, with the bent top notches due to the stress from the flashlights springs and the mechanical hits that naturally happen with them (my twin 5 year olds love to play with them, with the occasional bump and fall).
To me, AAs and AAAs equally leak.