In any case, the difference in illuminated field size between the 1.83" abd 2" is not huge.. about 0.1" off-axis for both the fully illuminated and the 70% illuminated zone.
I ran the numbers thru NEWT and I'm getting different results that what you've stated for some reason. With a 1.83" secondary, I'm getting a field diameter of 1.18" at 75% illumination and 0.44" at 100% illumination. With a 2.14" secondary, the numbers are 1.52" and 0.80" respectively. So, it appears to me that there is a substantial difference between the two secondary sizes as far as field illumination is concerned.
I have a 2.14" secondary on my 10" f/6, which is preferable to me because I enjoy deepsky wide field observing as well as planetary observing and am using at least one 2" eyepiece. The 2.14" secondary yields a CO of 21.4% which seems very acceptable to me. I've looked at a number of designs in NEWT and concluded that unless you don't care about edge illumination at all, it is pretty difficult to design a Newtonian with a really small secondary, at least in a smaller aperture scope with an f/ratio less than f/8.
I think the difference you and I get are attributed to two factors.
1. I was comparing a 1.83" with a 2" not a 2.14 inch.
2. I was look at how far off axis the fields were at 70% and 100% illumination.. so we have to multiply those results by 2 for them to be directly comperable.
Other differences can be accounted for different assumptions about the outside tube diameter and the focuser height.