Well I ve always wondered and my Kaplan page 512 says that the binding energy of 3H3 is greater 8 482mev than that of 3He3 7 711Mev by 0 711Mev And it attributes the lesser binding energy to the coulomb repulsion And that would indicate the 3He3 nuclide to be the larger And since a deuteron would repel a proton with a greater force than a neutron it s more logical and more probable to accumulate an additional neutron and the question then becomes as to why 1 of 2 nearby neutrons would want to become a proton