Different lar bro,Ichigo already had a hollow inside himself.When he "died", his hollow took control of his body,hence, the appearance of a full hollow,"Vaste Lorde" alike figure.His hollow probably got suppressed when Ulquiorra cut of his horns resulting in a huge explosions, and then Ichigo "came back"

Aizen's case is different.He was a pure shinigami(as we currently know), and he doesn't have any hollow inside his body nor was he hollowfied in any way(again assuming that he wasn't initially hollowfied which he mentioned before).The cocoon ,through my understanding, allowed him to incubate while he was turning into a hollow.Yes, he retained his shinigami body probably he thought full hollows are fugly .The eyes you see are similar to Ichigo's eyes right when he puts his mask on and him mentioning he'll eat Ichigo up,certainly shows that he is indeed hollow already