My friends and I have used the saying to make fun of countless chebby guys and i would like to know who to thank for the wonderful saying...
It's Spanglish. In Spanish, the pronunciation of B's and V's sound so much alike they are often interchanged in use.
They didnt cross the border, the border crossed them... "They" were here first. (Wife's a 13th generation Californian)
When I was in the army my car broke down on I-25 between Denver and Colorado Springs and I was given a ride by some cool mexican guys. They were totally toked up and we laughed all the way to my base. They called their 49 chevy a chort (short) or chebby and by the time I got off I was talking just like them, definitely pre-Cheech and Chong (1962).