The last 10 exports are stored for later download in the Export Manager. The Export Manager utilises browser storage space to hold these exports. The amount of data a website can store is subject to it's "storage quota" On chrome this roughly works out to 7% of free space of your hard drive. So if you have 60GB of free space a web app can store up to 4GB of data on your PC. Safari has a hard limit of 50MB.
You can view your storage quota and usage in the "about" dialog ( bottom of the main menu ). You can view the export manager via menu > view > export manager.
If you fill your storage quota then exports are only held by the export manager until you close Construct, as it cannot save them for later. NWJS builds are by their nature quite large, and the NWJS frameworks are also stored in browser storage ( using up more storage space ).