Actually the BX weighs 1200 lbs, and since to my knowledge IT is the ONLY sub-compact in existence this makes the question easy, if it is not a BX it is a GT or LT or some other strange device or it is a compact or larger.
Compact tractors I think it is stated in Muhammeds book are tractors less than about 50 horsepower. In my thoughts the JD4600 and the Kubota L4610 are about the end points of the compact, anything larger is no longer a compact utility tractor but a utility tractor.
The BX the only sub-compact you say?!, well the new and very expensive Cub may qualify and the new JD4010 may qualify while the new JD400 and 500 series are still lawn care devices with attachments and tractor styling and some tractor features.
I think Kubota screwed up when they said the BX was a new class of tractor, a sub-compact, because now everyone wonders what a sub-compact is. Actually, in my opinion they should have stated the BX is a small compact tractor. Another way of looking at it and I think it is Kubotas thought as well is that the sub-compact is a small utility tractor with some GT features such as a low, wide stance and excellent mowing capability and manuverability and stability and is user friendly combined with the power, durability, versatility of a compact. Soooo, will the real sub-compact please stand up.
