Articulation issues, which have been well documented (Kent & Vorperian, 2013), state that macroglossia, or an enlarged tongue was believed to be the cause for articulation issues in subjects with Down syndrome. Guimaraes, Donnelly, Shott, Amin, and Kalra (2008) concluded that it is not true macroglossia, but rather the overall mouth size in an individual with DS is smaller. The mere fact that there is little correlation between comprehension and production suggests that articulation issues are rooted in physical anatomy or motor control.
Guimaraes, C. V., Donnelly, L. F., Shott, S. R., Amin, R. S., & Kalra, M. (2008). Relative rather than absolute macroglossia in patients with Down syndrome: Implications for treatment of obstructive sleep apnea. Pediatric Radiology, 38(10), 1062-1067.
Kent, R. D., & Vorperian, H. K. (2013). Speech impairment in Down syndrome: A review. Journal of Speech Language Hearing Research, 56(1), 178-210.