That's right - "slightly bigger and totally incompatible" sums it up to a T. I had a school friend who had such a machine and kept going on about how much better than it was than the Philips, which was probably true, c.f. Betamax v. VHS...
Thanks for confirming that is was Grundig.
The Elcaset I well remember; a nice idea but much too late. It gets a mention in Wikipedia's list of commercial failures at