Even assuming quadruple redundancy, seven nines uptime and resilience to extreme conditions and cosmic rays, this looks way way bigger than it should be. One explanation could be that the certification process for anything aviation related is so expensive and slow that we're actually looking at tech from 15+ years ago.
If you mean physically bigger than it is certainly true that avionics, like automative, uses larger components with higher ratings and avoids a lot of high pitch integrated circuits. This makes things easier test and likely to be more reliable. It also means you might have a single euro-card just to handle the input from a single airplane sensor even though in the non safety-critical world, we would easily mux them onto a single interface.