Aizen just evolved further when he was imprisoned. At the end of the fight with Ichigo it may let Aizen get weaker, so that Uraharas Kidou could kick in, but ultimately it has its own will and I don't think it wants to be imprisoned in Muken. So it again strengthen its bearer.
And do those seals work on his reiatsu too? The way I understand it his body is sealed, but his reiatsu is at max power. That would explain why his kidou is that strong. And if he really fused with kyoka suigetsu he is now even more terrifying than before.