Two registers: (1) plain noun "hillock" — small natural elevation, often used metaphorically for piled-up things (
一小山的衣服 "a small mountain of clothes"); (2) proper-name reading
小山 = Japanese
小山 (Koyama / Oyama). In subtitle data,
小山 most often shows up as a Japanese character's name —
小山雅治 Koyama Masaharu, etc. Mandarin reading stays xiǎoshān regardless.