Can’t necessarily say I’d expect that to happen. Sonnb gallery stores more versions of each image than XFMG does. We only store an original image and a thumbnail but sonnb gallery stores, iirc, original, large, medium and small though I believe it did have the option to switch off the original version. But even with that enabled, converting to XFMG will have just imported the large versions.
What is the process you took to ascertain the size difference?