Hi Snowman

It's not that simple. Firstly the forks themselves have a rating that has to be checked - it varies in its own right in respect of distance from the forkface.
Secondly the carriage and its support chains have a rating - you will appreciate that the chains have a safe working limit which relates to the weight of the carriage and forks plus the rated load of the forklift. The rating of those chains may only allow 5000 lb to be lifted safely ever - any increase above 5000 lb could lead to premature failure of the chains. Note that these chains are subjected to high loads in travel with bounce, and an off-centre load could mean one chain is supporting the whole load.
Finally, there is the issue of forward stability - tip-over. A much heavier load means that under braking there is greater force trying to tip the forklift over - for most small forklifts a load equal to 40% of the rated load carried with its centre of mass 800 mm above the floor may cause it to tip over under full braking. A 200% load would cause it to tipover at a fraction of full braking!