I'm thinking out loud here but I think its more in the design of the curl cylinder where it meets the bucket. Larger tractors have a scissor type mechanism which I think keeps the cylinder away from the loader arm at further extensions and gives more of a curl/dump angle. Its just something I noted when we got a larger tractor and have seen it on most larger tractors above the 35 hp range.
Still, without redesigning everything a bigger bucket will do the same thing, with less spillage in the end. That is if your tractor can get a heaping bucket with the current one.