Orbital overlap and size "mismatch"
Small atom, big atom and small atom, small atom - shouldn't their
  overlaps be proportional


No.  Look at the figure below and let's consider s-orbital overlap, just to keep it simple.  Remember that these are spheres with the electron density distributed throughout the entire volume.  In the "small-small" case there is effective overlap of the electron density in both 1s orbitals; we would expect the resultant bond to be strong.  In the "small-big" case, the small 1s orbital involves a significant fraction of its electron density in overlap, but not much of the electron density in the large s orbital is involved in overlap.  In the "small-big" case the overlap of electron clouds is ineffective and any resultant bond would be weak.