This test app has now been released: https://portableapps.com/news/2019-08-26--gimp-portable-2.10.12-3-released
I'm posting a test of a new dual mode GIMP Portable build with both the 32-bit and 64-bit binaries. It's configured to upgrade your existing GIMP Portable install so be sure to make a backup first. It should preserve your existing plugins, brushes, patterns, palettes, scripts, etc, but there could be issues as the way GIMP Portable is upgraded has changed.
Install: 1.2GB - 1.34GB
The reason for this change is two-fold. Primarily, the 32-bit build of GIMP has had issues for about a year. It will crash on Windows 7 32-bit when you try to do just about anything. We'd previously used a GLIB patch that fixed this issue but the last version of GIMP this worked with was 2.10.6. Secondarily, the 64-bit build allows you to use 64-bit plugins as well as 32-bit plugins since we are including compatibility mode. Thirdly, there will be a small improvement in speed of some operations, but it will be unnoticeable in everyday use.
As to size, this is a much bigger package than the 32-bit built. This is due, firstly, to the fact that the 64-bit package is much bigger (910MB vs 650MB) just by itself. Including 32-bit compatibility to ensure no one loses access to their plugins increases that install size to 1.15GB. If we'd just had installs of full GIMP 32-bit and GIMP 64-bit, the install size would be a whopping 1.79GB. But, thanks to some creative file movements of the shared pieces between the two versions, the actual install size is shrunk to 1.34GB, only about 190MB larger than a 64-bit only package. Choosing not to install the additional locales as you install will save another 124MB bringing install size down to 1.22GB.
Note that you will need to install your plugins separately for GIMPx64 and GIMPx86. In App you will either see a gimp and gimp64 directory (if you ran it in 32-bit mode last), in which case the gimp directory is the 32-bit version, or you will see a gimp and a gimp32 directory (if you ran it in 64-bit mode last), in which case the gimp directory is the 64-bit version.
Please try this out on its own and with your current install being upgraded.
Test 2 fixes an issue with upgrading old versions and being able to update itself.
Test 3 fixes an issue where it broke itself in the process of installing.