Using your own words, you have a concern rather than an issue.
My advice is simply don't worry about it. It's remarkably common for people to take their working system and try to apply some preventtive maintenance to it and inadvetently turn it into a non-working system! Just look through the kind of issues raised here to see many examples.
Aperture is designed to handle the volumes with ease and if the current size is not presenting any actual problems for you, I'd just let it get on and do its thing.
Having said that, storage management and planning is important, but you aleady have the lions share of that side of things under control with the use of referenced files and external drives.
The actual library is (among other things) your index into your external drives and the images they contain.
Thumbnails and Previews are the visual element of that index used for browsing and sharing your collection.
Thumbnails are used for browsing and are not optional. So previews is where you have the most opportunity to control the size.
However, keep in mind it's a trade-off; make smaller or lower quality prieviews to reduce the space they take up (in preferences), and they potentially become usuable in fewer scenarios. Elect not to keep the previews for some (or all) images, and you lose the ability to share them through iLife/Media Browser.
Previously I had a smaller internal hard and so needed to make this trade-off, and my approach was to only keep previews for my top rated images, as these were the ones I would be sharing in slideshows and online. However being a pro, I doubt this kind of trade off would work for you. It needs to be tailored to your own usage, for example perhaps keep previews for all projects for 3 years along with those that you elect to reperesent your portfolio.
Andy