I know I'm a bit behind in posts but I haven't seen this comment on the net.
  According to my understanding a baby should double birthweight by 4-6
months.    For a 6# 3 ounce baby this would be 12#6 ounces by 4-6 months.
 The current weight is double.  If the baby was born in the 10% range and is
still in the 10% range it hasn't dropped any percentage points regarding
weight.  Why is this considered a big problem?

If mother is tiny (is she?) then is it realistic to expect the growth of
another size person.

Case in point.  My youngest weighed in at 5# 9 oz at birth.  Full term baby
17.5 inches long.  At one year she was around 16.5 lbs.  I was harrassed from
6 months on to put more  weight on this child.  At 13 she is still under 5'
tall and weighs a whopping 72 lbs.  All my attempts have been futile.  When
we force the issue the weight may more closely match the charts but perhaps
we have also caused lifelong eating disorders and weight problems.

Denise Parker, BA, IBCLC (La Crescenta, CA)