Yes it is difficult to compare eras, the game has evolved so much over the decades.Jon Morris was 245 lbs. not that it matters all that much, but compare to Jason Kelce @ 295 lbs.
An aside it is difficult comparing football players of different era's as the game, physicality and size has changed dramatically over the years. But Hannah, Gray and Armstrong were three of the all time best.
In my opinion…For an all time list, the only metric you can use is how a good a player was in their era under that era’s rules. It’s the only objective way to view it, albeit not perfect.