Illustrator is the place to do patterns. The new Pattern creation feature in Illustrator CS6 is totally easy and intuitive.
InDesign has nothing similar natively. There is a pattern maker plug-in from Teacup Software:
You can use step and repeat to create a grid of images.... It won't reduce the file size, though. Each instance will be a link (presuming you placed the original rather than pasting it), so you'll have a few bytes in pointer info for each one, but it isn't going to bloat the file any more than placing many different links. If it's a problem, make your pattern externally and link to that as a single image. In anay case, I don't think it will have any effect at all on the size of a PDF that you would export.
As it happens, Kelly Vaughn (also known as Her Royal Geekness) posted a blog post on Patterns In InDesign just the other day: http://documentgeek.blogspot.nl/2013/04/ignite-2013-making-patterns-in-indesign.html
While it may not be exactly what you are looking for, it does give you a good overview of what *is* possible using InDesign.