I bought my gelding about 10 months ago and all I use is csi pads. My saddle seemed to fit perfectly then. Now it feels like it's too narrow and wants to roll when I mount and slide back after a ride. I'm thinking he must have filled out more in the withers and shoulders from using the csi pads, do you think thats possible?... Has anyone had this experience?? 

Sure, it's possible, as is a change in nutrition under your ownership, or even training methods, etc. 

Yep, could be a variety of things including the pad.  If he had atrophy because of a poor fitting saddle before then relieving that pressure allows muscle to redevelop, along with different diet and training/conditioning.

that being said, as frustrating as it is that your saddle may not fit anymore, I don’t think topline muscle is a bad thing!  If you have a 3/4” liner on your CSI try the 1/2” liner, or do away with the CSI altogether and try a saddleright or a thinner 5 Star.  We find the CSI’s to be really bulky and only use ours when we have to in order to make a saddle fit as best possible.