I have a similar situation with my cable setup but had figured to ignore it due to the scheduled comcast digital-transition which never occurred. For my setup, the NTSC signal looks awful on all my TVs. I figure NTSC is supposed to look bad, but indeed not as snowy/bad as we both are seeing!
Previously I had figured not to care about a soon-to-be-orphaned NTSC signals since QAM/DTV over cable was working so well. But the orphaning didn't happen. Yet. So I'm caring more about NTSC until it does happen.
Yes indeed, a quality amplifier as well as attenuators ("pads") are a possible way to tune an RF signal such that better NTSC/analog picture is displayed. Before that I'd look at the cable itself andverify is all RG6 or better.
Also in your situation firstly I suspect it may be the TV size which cause the perceived crappier picture on the larger TV. A larger tv should "always" render a worse-looking picture for a NTSC TV signal, unless you back up to the distance where the picture subtends the same angle as for the small tv up close.
Sooo.... I recommend you consider to control that out of your datapoints as best you can, such as by swapping the two tvs temporarily just for a test - or at least move the small TV to the big-TV's cable to see how the picture looks. That test result should verify that there is a cabling problem to one of the rooms.
( Alternatively with the big tv maybe can you do "side-by-side picture" to shrink the picture to the exact size as the small TV, and see that the quality is just as good in that case? )
If after as much of the above as you want to consider adding a quality RF/CATV amp, searching avsforum for brands/sources would be a good next move to consider.
Also in some cases the cable company will cheerily provide you an amp, if you/they can demonstrate a low signal level entering your house.
Please let us know what you discover...