It's not about whether it is light weight or not, today's aircrafts are a lot easier to fly and handle than earlier generation ones, because of FBW and other pilot friendly features. The mig-21 was notoriously difficult to handle, and is a very unforgiving aircraft to fly - one small mistake, and you are toast. Today's aircrafts with new gen avionics will make sure that pilots don't accidentally take the machine beyond dangerous limits (though such safety measures can be overriden manually for special scenarios).
When the mig-21s entered service in the IAF, only veteran pilots with hundreds of flight hours would fly them, since they were one of the most sophisticated aircrafts in the world at the time, and the most sophisticated in the IAF. However, with the introduction of mig 29s, mirages etc later in the IAF, young pilots were sent to the mig 21 squadrons, whereas more experienced ones flew the more sophisticated aircrafts (later of course the MKI). That is why we had a lot of problems with mig 21s crashing, because rookie pilots graduated from basic/intermediate trainers to these mach 2 mig 21s that were very difficult to master, and required great piloting skills.
