I had a quick look on the internet for an answer, the first 'hit' was this .......
http://www.outsideconnection.com/gallant/ggp/wuzzyBig.jp g
( It is not a true representation of a guinea pig really ). The web site is by Max and Evan Gallant, two young guinea pig enthusiasts.
Phoberomys pattersoni was a prehistoric guinea pig relative which grew to the size of a cow.
3lb is a large guiea pig, 4lbs would win a regional competition
for heaviest guinea pig. The biggest I can find is a reference to was 4lbs in weight (almost two bags of sugar!)