I’m assuming these were base V8s, which of course put the lighter and smaller Plymouth at even greater advantage with its 230 hp 318 compared to Chevy’s 195 hp 283 and Ford’s even smaller 164 hp 260. FWIW, the little Chevy does pretty well under the circumstances to make up for its deficit in cubic inches and car weight. And the Ford was also the heaviest, by far.
Actually, the Chevy was only slightly heavier than the Plymouth, oddly enough.
Here’s the curb weights for these:
Ford Galaxie 4 door sedan: 3647 lbs
Chevrolet Bel Air V8 4 door sedan: 3345 lbs
Plymouth Fury V8 4-door sedan: 3265 lbs
The reason (in part): that poly wide-block 318 V8 was a lot heavier than the lightweight Chevy and Ford V8s.