Again, there is capacity and size. You want to lower the capacity but expect the size to be smaller as a result.
When you click onto "adjust folder size" button, the lower portion of the window which says "reclaim unused space" says....the disk image has xxx of data, and is xxx size on the disk. The second number is the actual size of the disk image on the disk, ie. this is the amount of data it uses on your disk.
In my case, it contains 2MB data and has 85 MB in size on disk...but this is because it is a test folder, so nearly empty, and therefore the discrepancy between the amount of data it contain and size on the disk is noticable. If I would add 200 MB of data into the protected folder, the size on the disk would grow maybe to 205MB so there would be 5MB of metadata.
Lowering the maximum capacity is not possible in Espionage, because it will not give you any benefit, you could do that by using disk utility but again, you would not gain much.
The reclaim unused space is another thing...say I added these 200MB of data, the size of the disk image on the disk grows from 85MB to 205MB. Now I delete these 200MB from the protected folder...the size on the disk would still remain 205MB because whereas the disk image automatically "grows" to accommodate more data it will not automatically shrink. But this is what you can do by hand if you click onto "reclaim unused space" button. But this makes sense only if you added and deleted some significant amount of data.
I hope this makes it clearer.
A friendly advice, if you are running so low on space on your boot disk, that 50 MB matters to you, then you will run sooner or later into trouble. I guess you are aware that OS X is creating memory swap files, the more applications you are running at once, or the more you run the Mac without reboot, the more of these pages you will have. If the OS X runs into situation where it needs the create a new swap file but it cannot things might get stuck.
Therefore I would suggest to leave at leat 5-10 GB of free space on the boot disk.
Let me know if you have further questions,
Rgds
Zsolt