It is true that Protocol Buffers are not optimized for low-mem systems. On the other hand, for preserving bandwidth and storage one can use lightweight compress

It is true that Protocol Buffers are not optimized for low-mem systems. On the other hand, for preserving bandwidth and storage one can use lightweight compression algorithms on top of general-purpose serialization library. (This is the approach that Google ended up using, AFAIK.) I think such approach is more flexible than making explicit assumptions on how much data is actually carried in integers.