"Did you know the thymus in a breastfed infant is twice as large as a formula fed infant?"
Although I have to wonder... Why are they still using formula babies as a baseline?
Perhaps instead it should say that "the thymus in a formula-fed infant is only half the size of a breastfed infant".
The thymus "educates" T-lymphocytes  (T cells ), which are critical cells of the adaptive immune system . Each T cell attacks a foreign substance which it identifies with its receptor . T cells have receptors which are generated by randomly shuffling gene segments. Each T cell attacks a different antigen . T cells that attack the body's own proteins are eliminated in the thymus. The thymus provides an inductive environment for development of T-lymphocytes from hematopoietic progenitor cells. In addition, thymic stromal cells allow for the selection of a functional and self-tolerant T-cell repertoire. Therefore, one of the most important roles of the thymus is the induction of central tolerance .
Source: http://en.wikipedia.org/wiki/Thymus 
T-lymphocyte subsets, thymic size and breastfeeding in infancy
PEDIATRIC ALLERGY AND IMMUNOLOGY , Issue 2 2004
Dorthe L. Jeppesen
We followed the changes in concentration of T-lymphocyte subsets (CD4+ and CD8+ cells) in peripheral blood and thymus size during infancy. Previous studies have found increased thymus size in breastfed infants. The present study analyzed the association between breastfeeding and the number of CD4+ and CD8+ cells. Two different populations of infants between birth and 1 year of age were examined. Study Group I: infants with a variable duration of breastfeeding. Study Group II: long-term breastfed infants. In both groups a correlation was found between CD8+ cells and the thymic index at 10 months of age. In Group I, infants still breastfed at the 8-month examination had a higher CD8% than formula-fed infants (p = 0.05), and infants breastfed at the 4-month examination had a higher CD4% at 10 months of age (p=,0.03). Group II showed an increase in the absolute number of CD4+ and CD8+ cells from 8 to 10 months of age; and a positive correlation between the number of breastfeedings per day at 8 months of age, and an increase in CD4+ cells from 8 to 10 months of age (p <0.01). In conclusion, a correlation was found between thymus size and CD8+ cells. Breastfeeding might have both a current and long-term immune-modulating effect on the developing cellular immune system. [source]