If you're maintaining the 3rd class medical, just fly the Cessna!
My own suspicion on the red line drawn on weight was to specifically eliminate most of the used aircraft in the GA fleet for Sport Pilots (and those flying under Sport Pilot rules).
When the FAA decided to buck the international standards on weight and add more on, it became somewhat an arbitrary number based on negotiation rather than on hard rules.
I can just imagine the talk in some conference room somewhere:
"So we're going to let folks fly without a medical and use their driver's license."
"Finally those guys with their fat ultralights are going to have to get some sort of training. Plus it'll hopefully get new people into flying - half the hours is half the cost, and most people don't fly at night anyway."
"What's the limitation on the planes again?"
"Well, we're working on the weight restrictions; Europe's is too low."
"Wait, Private Pilots who don't think they can pass the next medical check can fly under Sport Pilot rules."
"Yeah, so?"
"Keep most of the current fleet out of reach."
"Why?"
"The last thing we need is old medically unfit guys flying the antique aircraft that make up the bulk of the GA fleet."
"Antique aircraft?"
"How many people do you know that drive a car that's thirty five years old?"
"Yeah, you got a point; the fleet needs to be updated and these old planes retired. Besides, if the net is too wide there'll be no incentive for anyone that flys daytime VFR to ever renew the physical."
Seriously, though, the Sport Pilot license and rule should be understood for what it is - a seperate category of license that fills a specific niche in aviation. The primary purpose was not to keep seasoned pilots who (for whatever reason) don't want to renew their physical in the air - it's a loophole in the regs that allows them to do so. A reasonable loophole, but ancilliary to the intent and purpose of the Sport Pilot license, and should be treated as such.
Too many Private Pilots think of the Sport Pilot license and rules backwards. Indeed, I saw a website for a flight school that trumpeted that they gave Sport Pilot training for those that know or don't think they could pass a Class III physical; a pretty irresponsible position to take, IMHO.
It's like saying that since one can't see well enough to drive a motorcycle they'll take a bicycle onto the streets instead.
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I think a better solution would be to take a cue from the DMV on medical constraints and restrictions.
The reason Sport Pilots don't need a flight medical is the fact that the planes are simple, basic aircraft being flown in daytime VFR only. No IFR. No night time. Fair weather flying in two seaters that don't leave much of a smoking hole. Un pressurized cockpits so stay below 10K feet...oxygen use is not simple, basic aircraft stuff.
Now think of all the skill requirements heaped on the Private Pilot. Even if he never flys at night, IFR, or over 10K altitude, he's always got to be medically qualified to do so in the gov't's eyes since they don't know what he's going to do. It's like having a HAZMAT CDL and having to always meet the requirements for driving a truckload of nuclear waste even when he's driving the family sedan for some ice cream.
More reasonably, the FAA should put restrictions rather than simply pulling the whole ticket. Let's say a pilot's vision is getting a bit iffy or they start getting concerns about the stress of a high workload on the pilot because of a minor irregular heart beat or blood pressure. Fine. You still get to fly, but it's now Daylight VFR, Single Engine Only (for examples). Or maybe it's "Flight duration limited to four consecutive hours," or whatever.
No wonky loopholes, no "you're medically unfit to fly a Cessna 150 worth 25K but you're just fine to left seat a Skycatcher worth 125K" garbage.
Remember, the key to the medical rules for Sport Pilots is the lack of complexity of the aircraft and how it will be used.
The FAA is being lazy on this score. Rather than deal with the issue head on they're encouraging the stupid loophole.