From thermalright.com; SLK-900a:
L96 x W59 x H44 (mm) - Top, without fan
L34 x W59 - Base
SLK-800a:
L87 x W56.4 x H48 (mm) - Top, without fan
L57.5 x W56.4 x H48 (mm) - Bottom
L25 x W56.4 - Base
2-3mm wider, nothing else. The only thing that would help is surface area, but the difference is minimal here too, so not that large of a difference in cooling capacity.