It's very ambiguous to me what this option is for ... but it has no effects to tga files (just tested this). I think it's about Photoshop files ... not about all file formats.
Furthermore I guess this option is different for Mac and Windows (I've read something about this in the Photoshop help), but because I know exactly nothing about Mac's I don't know what's different.
Secondly, the tga file format itself specifies a thumbnail. (tga 2.0 at least)
But the thumbnail's max size is 64x64 pixel. (which means the max size of a thumbnail is 16 kb)
Not very helpful at all.
But I'm still think that's not a Photoshop or thumbnail problem.