Without any clinical infomation, there are a number of possibilities for this film. Some sort of developmental problem with the right lung perhaps, or interruption of the right pulmonry artery. However, if it transpires that there is an ejection systolic murmur or if the tentative clinical diagnosis is ASD, then you can call it as valvar pulmonary stenosis. Post-stenotic dilatation of the left pulmonary artery, with a normal or some times (as here) small right pulmonary artery, is a classic appearence.