I've never seen a petrol engine powered roller of less than about 2 feet diameter, and that 2 foot one weighed about a ton and was a professional item for rolling cricket pitches (long, long ago - it had been retired from use in 1958, and replaced by a bigger one with an electric motor). I've seen an unpowered heavy concrete roller only a foot diameter, and that was barely usable due to sinking into the ground (it weighed about as much as my old 20" cast iron roller). So, a wild guess: because small heavy rollers are really difficult to drag over soft ground, someone may have put an engine on one rather than increase its diameter.
