I once handled a very delicate, Hawken squirrel rifle that was extremely light, extremely slim, extremely delicate, and well balanced. Even the slightly swamped barrel was light in weight. The lock was a small, delicate looking piece of work proportional to the size of the gun. I almost bought it, but would have had to put the kids up for collateral. The wife was not amenable to that, so I had to pass.
So, yes, building an extremely slim, delicate gun can be done, though I suspect that getting the proportions and balance correct would be challenging. A real slim barrel would be a must. A 40 in an 38", A wt barrel might do the trick, but a smaller bore might require a custom barrel, proportional to the bore diameter, for balance.
God bless