By what mechanism does Risperidone swell breast tissue?
Risperidone prevents dopamine from acting on the pituitary gland by blocking dopamine type 2 receptors. One of dopamine's effects on the pituitary is to suppress prolactin production. In patients taking risperidone, prolactin production thus increases and prolactin levels in the body are higher than normal. In younger males, this can result in abnormal growth of breast tissue and even milk production. 

See e.g. Melkersson, K. I. (2006). Prolactin elevation of the antipsychotic risperidone is predominantly related to its 9‐hydroxy metabolite. Human Psychopharmacology: Clinical and Experimental, 21(8), 529-532.