yeah i think its the friction causing discolouration...
i'm not fat but i happen to scar extremely easily, even a scratch will scar my skin, and last winter my skin got kind of frostbitten/dry/traumatized from wearing jeans in minus 30 degree weather, and i got some friction on the seams and it left marks on my legs for a couple of months where the seams were. no they weren't tight jeans.
All we need is to get a good exercise...The right way of exercising so we can achieve better results... Some people feel good and productive if they look and feel good...then so be it..whatever works...whatever makes you happy... If you think confidence can be achieved through making yourself beautiful and fit..then go for it!