The rear flange of the receiver is .145" (small .145"). The thickness of the receiver is 1.025" (small .950"). The front flange of the receiver is .450" (small .270"). Lastly, the height of the front of the receiver is 2.240" (small 2.250"). Thunk! Thunk!.........that's me hitting my head on the wall. Shoulda kept my lightweight, but.........sigh!
The 1881 "lightweight" was built on a large frame Marlin, not the small framed .38-55 and .32-40 calibers. The thing that really jumps out at you, when looking at the lightweights is the forearm flange. That's the area that's raised between the loading gate and the forearm, where the receiver transitions to the forearm. This flange is noticeably narrower on the lightweight, yet not as narrow as the small frame.
To confuse things more, people often get the rebated frames and lightweights mixed up too. Both a very scarce, but the rebated frame is stepped underneath the frame, between the trigger guard, and the forearm. Only found in the first 500 or so 1881's.