_{Community Wiki answer generated from a comment on the question by Sean Gallardy}

I've seen this when the backups are large, the cluster size is small and the file grows. You can try using the largest NTFS cluster size available to format the volume.

There are also hotfixes that may help:

A heavily fragmented file in an NTFS volume may not grow beyond a certain size

It may also help to use trace flag 3042.

To allow the backup file to grow only as needed to reach its final size, use trace flag 3042. Trace flag 3042 causes the backup operation to bypass the default backup compression pre-allocation algorithm. This trace flag is useful if you need to save on space by allocating only the actual size required for the compressed backup. However, using this trace flag might cause a slight performance penalty (a possible increase in the duration of the backup operation).

_{Community Wiki answer generated from a comment on the question by Sean Gallardy}

I've seen this when the backups are large, the cluster size is small and the file grows. You can try using the largest NTFS cluster size available to format the volume.

There are also hotfixes that may help:

A heavily fragmented file in an NTFS volume may not grow beyond a certain size

It may also help to use trace flag 3042.

To allow the backup file to grow only as needed to reach its final size, use trace flag 3042. Trace flag 3042 causes the backup operation to bypass the default backup compression pre-allocation algorithm. This trace flag is useful if you need to save on space by allocating only the actual size required for the compressed backup. However, using this trace flag might cause a slight performance penalty (a possible increase in the duration of the backup operation).