baby grand piano vs grand piano

baby grand piano and grand piano both are nouns.

Word	Noun	Adjective	Verb	Adverb
baby grand piano	Yes	No	No	No
grand piano	Yes	No	No	No

As nouns, grand piano is a hypernym of baby grand piano; that is, grand piano is a word with a broader meaning than baby grand piano:

baby grand piano: a small grand piano
grand piano: a piano with the strings on a horizontal harp-shaped frame; usually supported by three legs

Other hypernyms of baby grand piano include grand.

baby grand piano (noun)	grand piano (noun)
a small grand piano	a piano with the strings on a horizontal harp-shaped frame; usually supported by three legs