I've no clear idea, but usually when it's increased the texture sizes plus its quality, like if it's vector, the object where it's applied and mapped will look more detailed. Instead, if the quality isn't increased, it rests the same, like nothing was changed.About the pixlerized textures, anyone knows why even 1024x1024 textures (the original HD textures I made) look so pixlerized?
However, the game engine render counts too. Also, make sure to don't have the alpha channel when it's not needed, it decreases the colors look quality. Save the textures in 24 bit to don't have the alpha channel (32 bit means have it).