what happen in the data folder?

Hi, i've tried the portable version 4 and i use it with a xampp intallation on my usb key 8gb. before the to install on the key there was a 2.5gb free. in the install folder of heidi there the HeidiPortable.exe and two folder App and Data. in app folder i found the heidisql.exe and i the data there a registry folder. so for to start the heidisql i doubleclick on the first exe i found: heidisqlportable.exe.
then appear a splash screen, then disappear and start the program with the connection window. that ok.
after some start, the pen key was full, and the data folder grown to 2,5gb!! so nothing else works from the usb folder. i've tried to delete the data folder content and to restart from the executable portable. then data folder grown to 87mb and application doesn't start. then i tried the heidisql.exe in the app folder and now start.
my question: why the data folder grown to full the key? why data folder must contain 87mb of registry? it's a kind of bug?
thank's
Giorgio