Comparing ergo scores of athletes whose body weights have a large variance is one of those difficult biannual tasks for coaches.
Power vs. weight ratio is an important consideration in coaches’ selection criteria for crews. A larger athlete is reasonably able to produce more power on a rowing machine. But when a larger athlete sits in a single scull in the water, racing a lighter athlete, the larger athlete produces more drag due to the level at which the scull sinks into the water, and therefore requires more power to equal the lighter teammate.
Therefore, the best selection method is to use single sculls on water (perfectly still water, that is). But on land, if using a Rowperfect, the 2k test will produce scores that are aligned with on-water times and are weight adjusted, and boat class adjusted. If you are using a concept2, you can use this calculator.
Or, if you want to see comparisons in a spreadsheet, here is a tip from Dutch coach Bert Cocu, including formulas.