The idea is nice, but I am not sure if the filesize of the track decreases with this rectangle.
example:
When you place no blocks around your map, then there shouldn't be an information in the gbxfile like "x=4;y=10;z=3 no block". Instead the gbx file should just show nothing " " about these coordinates and TM interprets them as no blocks.
I am really not sure, but that's the way a gbx should work, because otherwise the filesize has to be the same on an empty map or a full map. Since the only difference is the name of the block, which replaces the 0 (or whatever). Only the rotation of the block would be an additional information (bigger filesize). But I don't think, that it's making such a huge difference
So if you have now this rectangle, then it cuts off all the empty blocks around. But (if my assumption is right^^) a gbxfile does this automatically but just showing nothing " ".
That's why I think it's not necessary for filesize reduction.
You understood it ?
The only advantage could be ( as tcq already wrote) to build maps with a different basemap.
You have not a solid place to build in, but instead you can change the width, length and height. Only the overall number of places you can build blocks in should have maximum.
For example the usual space is 60x60x20. The overall amount of blocks is: 60*60*20=72000. So if you want to build a longer map than 60 you are able to change the 60 to 100. So you have a size of 100x60x20. But the overall amount if blocks would be too big. So TM should reduce the other 2 numbers. One possibility could be 100*60*12=72000 or 100*40*18=72000. Also a very flat map could be possible: 268*268*1<72000 (=71824 to be exactly

)
This would really be an advantage I see.
But there is still one problem with the landscape around the map. Around the map the landscape has a sold scale. For example the Stadium at stadium. You cannot build a 268*268*1 map in there, since the stadium isn't big enough...
what a long post 