To change how much space a thing takes. Resizing alters the fit and not the thing.
Re- (Latin, again) attached to size, from Old French sise, a clipping of assise (an assessment, a fixed regulation), from Latin assidere (to sit beside, to assess): ad- (to) + sedere (to sit); to resize is to sit down again with a thing's measure and set it differently.
To change how much space a thing takes. Resizing alters the fit and leaves the thing itself untouched, exactly as the root's reassessment only ever changed the measure, never the measured.