Increase dmg capacity

Like Kappy says, there is no way to enlarge disk images unless you used the "sparse bundle" type of image.

Take a look at the "image format" options in Disk Utility (after doing a "File" then "New Image" in Disk Utility) when creating a new image to see the sparse bundle option. Don't forget to select the encryption option of encrypting it if you wish that.

So this would create a "logical" disk volume that is physically a special file on the physical disk that you created it on, and that logical disk can be of any logical size (even exceeding the physical size of the volume it's being created on).

Then later you can use the "Resize" option in the "Images" menu of Disk Utility to resize the sparse bundle.

Also don't forget that you can technically create a larger sparse bundle that could physically exist on the drive you created it on, but if you attempt to fill it up with stuff that would exceed the physical size of the volume it's created on you will get an error.

So to solve your problem, you could create another sparse bundle encrypted image, copy the stuff from the other image and you would be where you wanted I think. Make sure you have the room on the disk that you try to create the sparse bundle on that has enough space to hold what you want to copy to it.

Good luck...

Unless you created a sparsebundle you cannot enlarge a .dmg file without creating a new one of the desired size and transferring the content of the smaller one to the larger one.

What is the purpose of the encrypted .dmg file?