Picking up on Wildeblood's comment, and having now downloaded and studied the OXP, some observations.
Firstly you should not have to make the diffuse_map that big! A 2048x2048px image should give you all the detail you need, but as Wildeblood suggested you might want to make the 'master' bigger to get the fidelity of detail, and then reduce it. Currently your image file is physically huge, but with very poor definition.
Oolite's own in-built textures are 512x512px but they are fairly basic, so 1024x1024px or 2048x2048px are good sizes and these won't over-tax computers with limited graphics abilities.
Big files are un-desirable for many reasons. They obviously take longer to download, and for those with a weak connection or a tight download limit they may not be worth downloading at all.
Keep the image relatively as small as you reasonably can (consider 2048x2048px the maximum for normal use), and make sure the file is as small as possible. For instance you can remove the alpha channel which is often redundant (I don't think yours uses it for anything) which will reduce your file from 7.6MB to 6.7MB. This is still huge, but the saving of nearly 1MB is worthwhile.
There is no need to add highlighting and shading to the image as Oolite will add shade etc depending on the position of the sun, and again all this (un-necessary) detail is making the file bigger than it needs to be and may cause the ship to look strange in-game. Imagine the sunlight falling on one of the shaded sides. This will leave the game trying to light the shadows and shade the brighter parts!
Also, it is not normally considered good practice to 'burn-in' lighting detail such as you have done with the spotlighting. It won't look natural, and of course once there it is very hard to remove or alter. This is best handled by an emission_map, which will give a natural look to spotlighting. I have discussed this here
on the screenshots thread.
Another little trick I use is to make the 'background' (ie the un-used parts of the image file) black as plain black is very un-demanding and consists of virtually no 'data', and therefore keeps the image file size as low as possible.
, the friendliest Gourd this side of Riedquat.