How to prove greedy algorithm for number partitioning? the partition problem (or number partitioning1) is the task of
  deciding whether a given multiset S of positive integers can be
  partitioned into two subsets S1 and S2 such that the sum of the
  numbers in S1 equals the sum of the numbers in S2.


There is a greedy algorithm for this problem:


  One approach to the problem, imitating the way children choose teams
  for a game, is the greedy algorithm, which iterates through the
  numbers in descending order, assigning each of them to whichever
  subset has the smaller sum. This approach has a running time of O(n
  log n). This heuristic works well in practice when the numbers in the
  set are of about the same size as its cardinality or less, but it is
  not guaranteed to produce the best possible partition. For example,
  given the set S = {4, 5, 6, 7, 8} as input, this greedy algorithm
  would partition S into subsets {4, 5, 8} and {6, 7}; however, S has an
  exactly balanced partition into subsets {7, 8} and {4, 5, 6}.


But, I don't know how to prove "This heuristic works well in practice when the numbers in the set are of about the same size as its cardinality or less". Can anyone help?
There is nothing to prove about the statement you highlight. It is an empirical statement.

In order to have something to prove, you will need to formulate this claim formally in some way or another. There are many options, including average-case analysis, smoothed analysis, and restricted instances (which is what the statement points at). Until you have done so, it is impossible to prove or refute the statement.

The example given in your source shows that, in fact, the heuristic doesn't always succeed. Perhaps it gives a good approximation in some circumstances. The statement, unfortunately, doesn't hint at a provable claim which has any chance of being true.