Get a marker and mark from the center of the prop...where metal meets metal of the prop shaft, straight out to the outside of the prop hub. Run it and get it to act up. Examine your mark. If intact a fluke of your operation or entangled weeds. If no longer intact, a slipping hub.
Rubber, pressed in hubs are designed to take a strike, lose friction connection with the prop barrel, allowing the prop to only deliver part of the drive applied to it and upon doing what you did (cut the throttle), reseat to a solid connection and function properly. If it is caused to slip too often you can damage the surface between the hub and barrel and it will will slip frequently forcing replacement of the hub (new hub pressed in), usually by a prop shop. For the cost of a new prop for that engine I wouldn't bother with a prop shop, just go to the top of this page and under props, order you a new one.
On a 20" midsection on a boat designed for a 15", you have plenty of water at your disposal, just a lot of lower unit drag which reduces your top speed and makes for reduced shallow water operation. Sooooo, unless you picked up some weeds around the prop, I don't see how you could cause prop blowout with your operation of the boat and slowing down wouldn't clear weeds. Were the situation reversed (lengths opposite) then it would be a problem at all speeds but idle.
If the length is a problem for you, considering your boat, just raise the engine mounting portion of your transom 5". 25 hp isn't all that much and wouldn't require a lot of effort.
