The part that doesn't make sense is a heavier weapon with parts that increase comfort for aim/use has worse recoil. But, in this case, that seems to not happen. That would be like saying a stock Honda Civic has better handling than one modified for racing. Weight alone should decrease recoil, as the gun can't kick as hard as a result. Ergo being worse only somewhat makes sense from a weight point of view. But if the gun is more easily controlled with the attachments, some of that ergo nerf should be mitigated because control has increased. through better grips and a change in the angle for the supporting arm.
The weight alone simply can't contribute to an amount of recoil reduction sufficient to be perceivable, not with the firearms we have in the game for sure.
As for the ergo, I really don't see why attachments matter all that much in terms ergo itself. The ergo, as we currently have it, seems to be just target/sight acquisition, and I think and underexperienced shooter wouldn't be as quick to lock sights on target, especially with a firearm they're not that familiar with. Things get worse and slower when you have any kind of magnification, and much better with red dots and stuff, of course, but I really doubt that some foregrips and other bells and whistles help you acquire target any faster - they're for recoil and bracing, they won't give your PMC the same skill as someone else, more experienced with the firearm in question, would have.
Attachments are important and stuff, but they shouldn't be the name of the game if we want to talk any semblance of realism in an FPS, really.