Get layer by uncompressed size with script

[Ofnuts]

The storage size of any layer is the number of pixels (so, width ✕ height), multiplied by the number of channels (from one for grayscale without alpha to four for RGB with alpha) multiplied by the number of bytes per channel value (one if 8-bit integer, to four if 32-bit floating point). When you export to Gimp recent format, you can get compression, which usually results in about one third of the original size.

The latest version, (Gimp 3.1.4) supports "link layers", where layers are the contents of an existing image file, so this could solve your problem more directly (but then as it is new it could have bugs...).