I think the complexity comes in with Part 25 and to a lesser extent, Part 135 and/or 121 Operations and "turbo-jet" power. So there probably is little or no difference in regulations between a Regional Jet or a FBW Biz jet and an A380. Additionally the complexity of regulations will also increase with how recent the design is - for example, you now have to certify Autopilot operation in Icing, which did not apply before.