[Kerry Irons]

Don't worry about it

I noticed today that the both tires, but especially the rear tire, has hundreds of small cracks (splits) in the sidewall. They are about the same length and are lined up around the fatest (the bulge) part of the tire.

It's called "crazing" and it happens to rubber. If you are not experiencing any sidewall deformation (bulges) then you have nothing to worry about. Just ride the tires until they are worn out. At your weight, 25 mm tires are probably a good idea.