A short story to explain one reason why the diameter limits exist:
Propeller Vibration: Issues, Examples, Consequences, by EPI Inc.
It specifically mentions your situation.
Note it states: "to be sure that a particular prop will survive the fatigue environment produced by a particular engine on a particular airframe". As yours is Experimental the question is what limits the prop shop is using? Did someone make up limits for your manual or are they using TCDS data or just assuming it is like a similar certified airplane?
The Type Certificate Data Sheet for my prop states blade diameters from 68 - 74 inches but the AFM for my airplane has limits of 72 - 74 inches. Performance may be a factor there as it is a certified airplane, I don't know that detail.