First find the contact point with the rip. If it is at the seam in the crotch, you probably just need to buy a bigger size or higher quality shorts. But, if it is happening lower down your leg, here is a thought:
I had a similar problem after getting a new bike and putting on a new saddle. Turned out the real issue was the saddle bag I put on the new saddle because I wasn't going to put a rear rack on my new bike and I hate carrying anything on my body. My chubby inner thigh was rubbing on the rough edge of the saddle bag. Achieving slimmer thighs would take too long - I bought a more narrow Topeak saddle bag and the problem went away.
The key was figuring out the contact point - my wife rode behind me and narrowed down the problem.