I can't understand why you would think there's a bigger gap. If you put 2 sheets of glass in runners that have the same width bit of plastic in between them it doesn't matter how thick the glass is. It could be wafer thin or 6ft thick the gap is still going to be the same cos they can only get as close to each other as the runners allow.
They can only be pushed apart as far as the runner allows as well. So if you have thin glass in wide runners then yes it will move more.
Sent from my ST18i