It's designed to be tough. The Soviets faced a fundamental difficulty when trying to craft a defense against the B-70: yes they could make a plane to catch a Mach 3 bomber at 70,000 feet, but how to shoot it down when Soviet air-to-air missiles were unreliable, lacked range, and were slower than the plane they were supposed to catch?
The solution: train their most ideological air defense pilots to fly a fast, strong, aircraft and in extremis - the B-70 would never be seen over Russia save in a nuclear war - command them to ram the bomber. So the MiG-25 is made mostly of steel, not aluminum or titanium save in critical spots, is propelled by two powerful and massively fuel-guzzling engines, lacks the swing wings of other Soviet aircraft of the period in favor of sharp edges and strength, and has a comparatively short combat radius.
MiG-25 fans should note that the only confirmed kill scored by this aircraft was when one overran the end of a runway and sliced through a school bus, decapitating most of its passengers. (ref: MiG Pilot)
It shot down a F-18 and a israeli F-15 (according to syrian sources)