If you look at the land grants in the DeWitt colony, they are much less worried about being equitable and more about claiming prime lands... Evidently, there were plenty to go around among the earlier colonists.
I think it may be a difference between the styles of Green DeWitt and Steven F. Austin, and also the kinds of settlers they initially attracted. The Old 300 may have been a bit more genteel than the earlier admittees to the DeWitt colony, who probably had more of a frontier background/were from less established areas initially. In other words, they picked lands in an orderly fashion versus a mad free-for-all to get the best lands. I may be totally wrong with that analogy, though. I know Austin brought settlers from his earlier speculations in Arkansas and Missouri as well, so it's not exactly like the Old 300 were afraid to pick up everything at the drop of a hat for a better stake.
It probably had more to do with the fact that more of the area in Ft. Bend county was arable farmland (read: flat with at least marginal soil), versus the rich alluvial valleys surrounded immediately by upland hills and prairie thought unsuitable for farming in Gonzales Co. That would explain the more regular and orderly shapes of the Austin grants, and the more topography-based surveys of the DeWitt Colony where they are compressed closer to the river.
The larger tracts on the map are areas where the Guadalupe/San Marcos river valleys are very wide, smaller tracts where the river valley is narrower.
I occupy a fair part of the Eliza DeWitt survey just east of downtown Belmont, Tx.
[This message has been edited by SWCBonfire (edited 12/2/2008 10:08a).]