Suspect that safety kit like squashy bumpers and airbags must have something to do with size expansion. Even so, it is a fact that today’s Polo is well larger than the first three or four marks of Golf.
Suspect that our need to have ever more things in a car also contributes to bloating – in 1976 my new mini had under its bonnet an engine, a gearbox, a couple of small electric things (alternator etc) and a radiator. A new mini today will have to hold all the above plus a couple of turbo units for engine, compressors and fluid tanks for air con, pumps, tanks and wracks for power steering, possibly oil-coolers and a whole lot of other gubbins which weren’t thought of when Sir Alec made the Mini.