SD cards have rarely exactly the same size, but when you make a backup, the backup includes the free space. There are tutorials where you can kill the free space and than backup. Sorry for my bad English
Just to cover all possibilities, I’ll note that there are a great many fake sd cards out there – the actual storage space and the reported storage space do not match (e.g. a 32GB card is actually an 8gb card with the header information altered to report the larger size).
It’s most likely the issue is due to the overhead on the image, which I’ve seen before, but it’s worth eliminating the slight chance the new card isn’t all it seems to be.
I have ran into the same problem. I use 16 and 32 GB cards in my projects, but now I create my images using an 8GB card which I then create a backup image. I can then copy this image to the larger cards. Never have an issue with the size issue this way.