Can extremely tight jeans cause dvt (blood clots) in legs?

Not likely: If your jeans are just tight in one part of the leg acting like a tourniquet it could theoretically cause stasis or slowing of flow which may predispose to dvt. How ever if it is uniformly tight like jeans most likely are, it will compress all the veins aiding in venous return. Some patients with venous disease are actually given compression stockings to help venous return.

Unlikely: Dvt cases are multifactorial, usually because of stasis of blood in the legs( e.G bed ridden patients), hypercoagulable disorders (e.G cancer patient or hereditary disease, oral contraceptive pills) or injury to the deep venous system. Unless your tight jeans are resticting flow of blood from your legs( cannot walk , swollen legs ) then they would not be a causative factor.

They shouldn't : As long as you are not constantly crossing your legs the y sort of act like compression stockings. If your legs feel heavy after wearing them for some time. You maybe impending venous return so take them off and see if your legs feel better.