In some places, Rashi translates בירה as a "מגדל [גדול]", a "[large] tower" (Bava Kamma 61b, Taanis 16a). In other places he translates it as a "בית גדול", a "large house" (Bava Kamma 22a, Gittin 55a). In still other places, he translates it as a "טרקלין גדול", a "large palace" (Makkos 5a).
Why does Rashi translate the exact same word differently in different places? If he just means that a בירה is a large building, why doesn't he translate all of them as just that - a בנין גדול?