I've using the built in defragger in Windows 7 every week and my incremental backups stay small. But if I get a commercial defragger, will my Macrium incremental backups increase in size? That used to happen when I used Acronis True Image Workstation years ago.
It could very well do, it all depends on the algorithms in the commercial defrag software and the extent to which it moves data around the hard drive. If the file system layout is changed extensively by the defrag software then you could see a mirror change in your incrementals.
I use the free Smart Defrag software. It seems to do a very good job. However it DOES cause an issue with the incremental size. Therefore I ONLY run this software between my last incremental and my next full backup. I do incrementals daily (EXCEPT the day of my full backup). I find that a defrag is only necessary once or twice a month. And that is only because I "feel better" when my drive is "tidy". In reality, defragging every couple of months is probably fine for most systems.
I watch TWIT network on my Roku box, and Leo Laporte who has been doing the Tech Guy show for over 20 years and really knows his stuff says that Defragging a modern hard drive today is a waste of time. He feels the same way about registry cleaners, which he says can cause more harm than good. http://twit.tv/
I love Leo - I used to watch him on TechTV. Many people share his opinion about defragging - others do not. Certainly it is useless and even a bad thing on an SSD drive. However I think it is still needed. There are pros and cons and it depends on how much free space is on your hard drive. Here is a good discussion of the subject.