When multiclassing in 5E you are basically weighing what upgrades you would miss out on against the benefits of adding a new class.
To build on this, I believe in 1st/2nd edition you considered the exp requirements for each class separately. I.e., if at Fighter 10 you dual-classed into mage, then it would only cost 2000 exp to level up from 1->2 in mage where it would cost 200,000 to get level 11 fighter. In 5e, leveling up is determined by your total level summed from all your classes.
Because of the exponential growth in exp, the equivalent multi-classed builds for a level 10 fighter are:
In 2e: F/M 8/8 -> You are close in power to your level 10 companions, made up for in versatility
In 5e: F/M 5/5 -> Your companions (and enemies faced) are overwhelmingly stronger and better than you in every way
