[Hallicrafters] capacitor question for cap whizbangs

[Nash4447 at cs.com]

I have a piece of electronic gear built about 1980 that uses a pair of 
6.8MF  100V block type capacitors. The caps are almost the size of a pack of 
cigs.

I recently purchased a pair of replacements marked with the same capacity and 
voltage ratings yet the things are just slightly bigger than a pencil 
erasure. 

What happened over the intervening 25 years that allows today's capacitor 
makers to achieve the same capacitances & voltage ratings in such tiny packages? 
Its really hard for me to believe that one will replace the other considering 
the dramatic size differences. 

Comments?

Glenn