Frames vary a lot, in geometry, bottom bracket placement, you name it.

So don't automatically assume that if you fit on a X size frame from one manufacturer, the size X frame from another manufacturer will fit.

It's a lot like buying jeans. I wear a size 10 in some brands of jeans, but I had the happy experience recently of trying on another brand in size 10 and I couldn't even pull them up over my butt.