What type of MWD you are calculating? empirical one or sand free MWD! Actually MWD has no absolute max or min value, but it depends on aggregate size distribution in different size fractions. Could I know what type of value you got, so that I can explain better!

MWD of the soil (an inceptisol of Indo-Gangetic Plain) under long-term fertilizer experiment with jute-rice-wheat cropping system at Barrackpore (determined by Van Bavel 1949 method) ranged from 1.04 to 1.89 mm. Application of both chemical fertilizers and farmyard manure increased the MWD of the soil.

Ref: S.P. Mazumdar, D.K.Kundu, A.R.Saha and D. Ghosh (2015) Effect of long term fertilization on aggregate associated phosphorus under jute-rice-wheat cropping system. Paper presented at National Seminar on Soil Health organized by Kolkata Chapter of the Indian Society of Soil Science at Kolkata on 8-9 October 2015. 

I agree with Avijit Ghosh that MWD depends on aggregate size distribution in different aggregate size fractions because it mostly calculated from aggregate size fractions. it may be also varied between soil types, cropping system, kind and amount of straw returned or incorporated.

Could you please give the range of MWD measured in your soils, specifying your soil type, field conditions and treatments? That will be very useful to other researchers. Thanks.

I got MWD of 2-2.4 mm, under double rice cropping system in inceptisol, have organic, inorganic and integrated treatments. 

Plesse see Sinha et al (2014) AJAR 9(2):285-293. Working with different cropping systems, they measured highest MWD of 2.81 mm under P.millet-wheat-cowpea due to dense rooting pattern & soil binding capacity of cowpea roots; and the lowest MWD of 0.70 mm under cluster bean-broccoli-onion cropping system due to presence of two vegetable crops with shallow and nonbinding roots.