This is possibly one of the most nonsensical remarks ever made on the internet.
Do you realise cars are made up of plastics and metals?
All you see is the paint (which is closer to a plastic than a metal). Saying one game looks like plastic while another looks like metal is completely nonsensical and AFAIK has never been substantiated.
Actually you're the one making nonsensical statements here.
Various materials and compounds react drastically differently to light, and it's perfectly reasonable to state that 'one game looks like plastic while another looks like metal' in this day and age. There are significant (hard to overstate this) technical hurdles, and with everyone looking to trim every single bit, it isn't unheard of for shaders (code used to help render different materials) to lack coloured/glossy/fresnel/etc information. Since the cars will of course take up a huge amount of screen space, any extra performance cost (remember, this is computed per-pixel!) will drastically increase.
Add in the fact that car paint is one of the more complex materials that can be rendered, even in offline rendering (movies, etc), and you've got a significant technical issue.
Look into conductive vs dielectoc material rendering, please.
This could be due to the fact that Demos tend to have reduced visual details in order to save demo file size.
File size has nothing to do with it.
Low amount of content = smaller file sizes.
Games do not work like videos.
It's not a linear thing either.
The 3D mesh/meshes is probably the largest piece of data for a car in GT5/6, by a HUGE margin.
The physics properties are fairly small; think of a txt file with everything about the car, it's still going to be tiny.
Next would be the shader; you'd need one for metallic, pearl, gloss, matte finishes. Like the physics info, it's fairly small.
Each material (think paint chip) would reference a shader type (metallic), base coat, top coat, glossiness, reflective values, etc - very tiny.