Weight of the Disc Harrow is the main factor in how it cuts, until its sinks to pan hubs. To a lesser extent, how the gang axles are set affects how it will cut. The third factor is pan spacing. Seven inch pan spacing provides much more float than nine inch pan spacing. More float, less penetration.
Disc Harrows, tilling deeply, resist with considerable draft force. So your tractor tire types, whether the rears are filled and whether or not you have 4-WD make a great deal of difference in what your tractor can successfully pull. In order to get good soil mixing you need to pull at a brisk walking pace, minimum. Soil moisture is also important.
The best way to discuss Disc Harrow for compact tractors begins with pan diameter, pan spacing, total weight, then width. You need at least forty pounds bearing on each pan to cut well with 9" spacing. Fifty pounds is better. Disc with seven inch spacing are pretty much smoothing, rather than cutting Disc Harrows. If your primary use will be smoothing plow furrows, seven inch spacing will leave a smoother bed than nine inch spacing.
Disc Harrow are measured for width with the gang angles flat, though the gangs will not cut set thus. As the gang angles are adjusted more aggressive they assume a > shape, therefor the width measured across decreases. So there is no "one" width measurement. Operating width will always be less than measured "flat" width.
A lot of what has been posted so far in this thread is just blah, blah, blah. Incomplete tractor information, incomplete Disc Harrow information.