I think you are working very hard here and working on things we would all enjoy, but there appear to be several tools that you are using or not using that end up putting your units at a mismatch with the CIV3 technology.
These planes end up looking like they do not belong in the game with the other units. Not because they are not any good, but because they do not seem to go with the other units visually and stylistically.
I am spending a lot of time working on these issues without producing any units because in the long run, units that do not fit these style issues will be replaced and thrown away.
I think the Smoker has pinpointed one of the issues with the type or rendering perspective you may be using or borrowing from other graphics.
I think that you also may be manually editting for shadow and shading and that ends up giving you the effect that looks like the shadows do not go with the unit image. The shadow for the Giant transport looks like a B24 shadow that failed Jenny Craig.
There are several shareware programs that will let you generate better shading and shadows once you construct a good wireframe model for your units. This will let you get more bang fro your buck and you can also generate some really cool images for other uses.
Good luck,