Midsize and full-size SUVs differ most usefully in how much space and capability they provide while carrying a real load. A full-size model may offer a more usable third row, more luggage room behind it, or a stronger towing configuration. A midsize SUV often asks less of the parking space and can be sufficient when most trips use only two rows.
These are size categories, not guarantees of construction, seating, or performance. Compare exact models and configurations: an extended-length full-size SUV is a different packaging proposition from a shorter version, and a two-row midsize serves different priorities from a three-row midsize.