I dont know that leaving more space on shelf will stop the frosting. Warmth is causing the frost to form - how its getting in is a guess at this point.
I have a frost-free chest freezer that i have to defrost every 6 mo or so because there is no way of keeping warm air ( garage kept ) out. ( see - kids above lol )
It may just be that you have to defrost every so often which, in an upright is a PIA.