In Kyiv the metro uses plastic tokens.
I have an example of the Kyiv metro token, and also one from Minsk. The latter is also plastic (though pink to Kyiv's green) but has a metal thread running through it that is exposed at two points on the rim. I presume this is to give a magnetic signature to the token to stop other plastic discs being used (though in a country where people religiously wait at WAIT signals even when there's no traffic for 500 metres in any direction, I find it hard to believe that anyone would be so subversive as to try to defraud the Metro!), but it might also serve to make the piece a bit heavier. I can't remember whether the Kyiv one also has metal bits.
On the practicality of getting light coins to register in machines -- the problems are alive and well in the UK. On numerous occasions I have had 5p coins stop somewhere in the bowels of a parking ticket machine because it's lost its momentum as it passes through the recognition device. Usually some percussive maintenance gets it going again. The problem is a mixture of weight and size. Large non-dense coins (such as East German 50pf upwards) aren't a problem because they're big enough to give them a good shove into the slot. Coins the size of UK 5ps, even when they're made of a denser alloy like copper-nickel, just aren't big enough to do this with; you rely on gravity to carry the coin down into the mechanism, and it doesn't take much resistance within the machine to stop the coin somewhere it shouldn't be. The DDR 10pf are on the small size to be reliably 'shoved', so perhaps to be safe they made the 20pf out of something denser. I can also quite imagine that East German engineering and technology wasn't necessarily up to scratch compared to modern equivalent machines.
