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This commonly occurs in two situations, both of which you can address. It may be that some of those files are huge, perhaps being virtual machine (VM) files for virtualization software like Parallels. If that’s the case, add the folder containing them to the list of exclusions using the Options button in Time Machine. As they change each time you use the VM, they’ll rapidly fill your backups otherwise. The other problem occurs when your Mac is backing up from an APFS disk to backups stored on an HFS+ disk. APFS can store sparse files, whose content is mostly empty, in highly compact form. As HFS+ doesn’t support the new file format, they have to be expanded to full size to be added to your backup. A few megabytes of file can then explode to several gigabytes.
In this case, the best solution, if you can’t exclude them, is to change to Big Sur’s new option of making Time Machine backups to APFS. To do that, you’ll need to be running macOS 11, and create a new series of backups on a fresh disk which Time Machine can format in its special variant of APFS.