I presume there is a slightly creepy reason to do with detection and traffic offences - but maybe that is just my paranoia - does anyone know the explanation?

My immediate thought would be that the rear plate has a badge for the Canton and Country, whereas the front plate doesn't - so theresno point making a plate the same size for less information (?)

...or am I thinking too Swiss about it?

"because".

You might have noticed that many continental licence plates have raised digits - which make photographing them easier

Which then brings up the question why the front plate doesn't have badges...

You will also notice that some rear plates are squarer with two lines and others longer with the text in one line. You can ask for which better suits your car at the Street Traffic Office.

I think it is how it was originally designed and since there is no-one in Brussels forcing continent-wide legislation on the Swiss, that's how it remains...