There are many reasons for babies to not weigh as much as the 'norm'. And for twins this becomes even more worriesome. My baby was born at 27 weeks weighing 2.2pounds. Generally, lesser weights mean more medical issues like under developed lungs, instestines etc. Emily was talking from a layman's point of view when stating that the 4 pounder was healthier. Babies with lesser weight may seem healthy at first but some develope NEC (necrotising enterocolitis) weeks after birth.
Some Babies lesser than two pounds may have no medical problems and some with more than two pounds may have a host of medical problems. Luck of the draw.