I've just checked and if I pick a Walk, in this case I picked the Lairig Ghru, click on the Route Map and I can zoom in to the full 1:25k scale, just as I can on the GPS Planner.
As I understand it, you have to be registered on the site, which you clearly are as you're posting, and you have to be logged into the site, which again you must be to be able to post. If you are not registered or logged in, you can still view anything but cannot post and the maps will not zoom down to the full 1:25k scale.
Might be worth asking on Help Forum, Paul Webster may know