While it is true that hybrids are sometimes larger than either parent, Rattus rattus and Rattus norvegicus are not known to produce viable hybrids. There is one anecdotal account of young being born alive, but the young either died or were killed shortly after birth.
A cat certainly has proportionately more of its weight in its legs, and a rat would have a proportionately larger body for any given weight. However, a 3 pound cat is still much smaller than the average house cat.
All I can do is reiterate that size is very difficult to estimate accurately for an untrained observer, and not that easy for even a trained observer. The animals you saw may have been unusually large for rats, and that “surprise” factor may have caused you to estimate their size as even larger than they were.
The giant rats I’ve seen, as early as 70s in Chicago, are not as fit as that African pouched rat. All of them were way obese with distended overall girth and have funny gait from carrying such a huge torso. Also they always have the distinctive hairless rat tail.
Yes, and that matches my very fat rat. He was more football shaped. Trust me, your first reaction would not be “a rat the size of a (very small) cat!” but “man, what a fat rat!”.