File size may be smaller due to image compression, but since it is exactly 3x more, it is also possible that Fiji chooses to store the image in 3 channels (RGB). You can try to change color mode to grayscale.
Check Volume module / Volume information / Number of scalars. If you see 3 there then it confirms that Fiji converted the grayscale image to RGB, needlessly tripling the memory usage.
When you resample your image to lower resolution then small details in the image are lost. For example, if you only had a few, isolated voxels that had the intensity value of 255 then it is very likely that you will not have a value of 255 in the downsampled output. You can slightly increase the quality by applying antialiasing filter (low-pass filter, such as Gaussian smoothing should suffice), but it rarely makes a practical difference (see this discussion for details).