Hmm, maybe you're right. I was mostly considering the parts needed for me (linear-replay) but I guess when you have OSC,Midi and all other kinds of integrations there's a lot more integration code weight.
Though from experience with my own tools, I suspect that in the end the UI code for the tool will outweight most other parts in the end.
Yes, there's definitely more UI code (roughly 20% of the total is engine code). But it's also a much, much easier code to write than the engine code which has to be threadsafe, has to perform no allocations as far as is possible, comes with various graph algorithms, etc
Though from experience with my own tools, I suspect that in the end the UI code for the tool will outweight most other parts in the end.