Mens Jackets are skin tight

I have been trying to make a decent men's jacket for a while now. My main problem is there is no "looseness" slider for jackets, as there is with shirts. The result is that my jackets always look too tight.

I have discovered a workaround. Create a baggy shirt and wear it under the jacket. THis stretches the jacket, but is there a better way of doing things?
I have seen baggy looking "puffa" jackets, but I assume that these are attached prims?

And finally, can I stop the zip on the front of the lower texture from replicating on the back? Or does the front and back of the lower part have to be identical?

Are you merely applying a texture, or did you make the shirt in Photoshop? If you're merely applying a texture, there's no solution other than to edit the texture yourself. If you made the shirt in photoshop, then yes there's no reason for the front and back to have anything you don't want them to have. If you use contrasting colors such as black with white -- i.e. the shirt being black and the jacket being white, it'll look better.

edit: Other than that, I'm not too sure. I shop quite a bit and you're right, most of the clothes is tight (lucky for me I like it that way).

They are created in photoshop. The front and back of the jacket's upper texture can be different. But as far as I am aware, the lower jacket uses only one single texture for front and back, so it has to be identical.

If you use LL's or Chip's templates you should be able to. I haven't done it myself (I only make shirts/pants), however I have bought an FKNY jacket that does such. Maybe it would we worth checking out their store to get a better understanding of how it's done

think the jacket layer needs a major reworking where possible I avoid using it because of the lack of suitable sizing options it should have had the shirt layer options. now I do more shirts as undershirts to leave the shirt layer free for jacket use or include both layers so the customer can pick.

The jacket bottom is basically just the upper part of the pants template. So yes the front and back can be different. I've seen some men's suits that use the skirt layer for the bottom and that too has a different front and a back. It looks like you've found out the limitations of clothes making in SL. The only way to adjust the shape is to adjust the shirt layer or add prims to the clothing.

I use either the skirt layer or floopy prims for the lower part of a coat. The jacket lower bit is sometimes useful for things like long waistcoats.