Increases effect of mass on vehicle speed

Was doing testing for a road roller (uses ten roller drums for wheels) - experimented on a roller I've found (51' rollers, so wheel bigness was a constant, done on v12 engine), I was changing width (w) and mass (m) of the roller drum, while jotting down the safe/maximum speed (sp) that was attainable:
w=200, m=750 kg -> sp 33/1239
w=200, m=1500 kg -> sp 33/1239
w=200, m=15000 kg -> sp 33/1239

w=60, m=750 kg -> sp 95/1239
w=60, m=1500 kg -> sp 95/1239
w=60, m=15000 kg -> sp 94/1239
w=60, m=1 g -> sp 95/1239

w=20, m=750 kg -> sp 197/1239
w=20, m=1500 kg -> sp 197/1239
w=20, m=15000 kg -> sp 189/1239

While a v12 engine is powerful, being able to blast a roller weighing 150 000 000 grams (10*15 000kg in the most extreme case) at 189 km/h safely is a bit too much. That and the fact, that increasing total vehicle mass by a factor of ten lowered the safe speed by 8 km/h (4%).