Why does TimeMachine generate large backups and is there a way to optimize them?

I have 2 Macs:

• MacBook Air (13-inch, Early 2015), 128 GB 10.14.2 (18C54)
• MacBook Pro (15-inch, Early 2009), 512 GB El Capitan (I think)

Both of them make backups to the same Time Machine (set up on a local NAS).
My issue is that MacBook Air generates large backups almost every time it connects to Time Machine - by "large" I mean over 1 GB, often several GB.

This seems unusual to me because, to my knowledge, I don't generate large files or large number of changes on it - I use it for general office work: typically email, calendar, some work with MS Office (Word, PowerPoint, Excel), browsing.

The other Mac (MBP) is used for similar tasks, maybe a little less intensively and it backs up more frequently (typically every day vs. every couple of days) but it generates significantly smaller backups, at least by order of magnitude.

The other difference between those 2 Macs that may play a role is the macOS, but both of them have highest version of the system available for given hardware.

How I can reduce the size of backups generated by my MacBook Air? If this is more or less normal behavior, please let me know as well.

This utility can show you what has been included in the last backup: BackupLoupe

If some things are being backed up that don't ever need to be, then you can add them to the exclusions list in the Time Machine Preferences section when you click the [Options] button.