Originally Posted by Fototim 
55-300 is a heavier and larger breast.
To me, this suggests more glass and better optical quality. I think it is that simple.
Tim
More optical quality maybe, but not more glass:
Pentax 55-300mm 12 elements in 8 groups
Tamron 70-300mm 13 elements in 9 groups
Pentax 18-250mm 16 elements in 13 groups
Pentax 50-200mm 11 elements in 10 groups