Weight limit is only one fit dimension
Whisker publishes a 3–25 lb range for Litter-Robot 4. Neakasa publishes a range extending to roughly 33 lb for M1 Plus. The higher ceiling is useful information, but it does not automatically make one machine better for every Maine Coon.
Long-bodied cats need enough room to enter, turn, posture and exit without brushing hard against walls or ceilings. Measure shoulder height and nose-to-rump length, then compare the physical design rather than relying on breed names in marketing copy.
Open top versus enclosed globe
An open-top machine removes the psychological and physical constraint of entering a globe. That can be valuable for a cat that already avoids covered boxes. The tradeoff is that open litter beds generally expose more litter surface to the room between cycles.
An enclosed globe can contain the litter area more tightly and may help with tracking or odor management, but the cat must accept the enclosure. A big cat that uses the machine awkwardly is a worse outcome than a theoretically better odor system it refuses to enter.
Check the room around the robot too
Large cats need approach space. A machine squeezed between a wall and cabinet can turn a generous opening into an awkward entrance. Leave enough clear floor for the cat to approach straight or turn naturally.
For small apartments, measure the machine footprint plus the clearance needed to remove the waste drawer and service the unit. The best robot is not simply the smallest footprint; it is the one that remains usable after you account for human maintenance space.
Bottom line
For an unusually large Maine Coon, open geometry and published weight headroom make Neakasa M1 Plus a logical model to inspect first. For a cat comfortably below 25 lb that already accepts covered boxes, Litter-Robot 4 remains a realistic option. The final decision should be based on the individual cat’s dimensions and behavior.