With $fit == true, the image is resized proportionally so that its _smallest_ dimension matches the width or height specified, NOT both.
For example, if you say thumbnailImage(400, 400, true), on an image of 1600x800, it will be resized to 800x400, NOT 400x200 as you might expect.
The solution is to compare the original image's dimensions to the specified dimensions, and substitute zero for the smaller dimension, and set $fit = false.
i.e.: thumbnailImage(400, 0, false) would resize that 1600x800 image to 400x200.