Cody and Abby (father's, sis's Yorkies) both gained weight after their spay and neuter. Abby gained quite a bit and had to be put on a "doggie diet."
She actually had an allergic reaction to the sutures used
and became ill and wouldn't eat anything for a bit.
Cody stopped marking in the house and whining when our dog (who doens't live there) was in heat, but he didn't start doing that until AFTER he was used as a stud.