A sparse image file is a single-file disk image, where unused blocks do not consume actual storage. These files will start out very small, no matter how large their capacity, but they will grow dynamically as new files are added. I don’t know if they will ever shrink when files are removed.
A sparse bundle is not a single file. It is a directory/folder full of files, each one representing a piece (I think 8GB) of the file system’s data. They are intended for use on storage devices that can’t handle (or can’t efficiently handle) very large files, including FAT and various network file systems.
The individual files within a sparse bundle are created and deleted as the storage used by the disk image increases and decreases (as files are added/removed). The space consumed may be less than the configured size of the image, but it may be larger than your files, since storage is allocted as large chunks (those 8 GB files).
But note also that if the image (of any kind) is formatted APFS, then it supports several features that may make a file consume less storage than its size would imply, including:
- Sparse files. Where unused blocks are not stored. They read as zeros and will cause storage to be allocated when they are written.
- Copy-on-write duplication. If you duplicate a file in the Finder or with a tool that uses the corresponding APIs, no new storage is allocated until you start writing to one (or both) of the files. So the storage blocks that haven’t changed are shared by the two files.
- Hard links. Where two or more directory entries refer to the same file. These are used in many places throughout macOS. These are actually features of all Unix-like file systems, including HFS+, and all Linux file systems (ext4, zfs, etc.).