The larger one is probably just older then the seller claimed. 2.5" is the average size of an adult betta (6 month+ betta), the others are still teenagers. Senshi (my avi) is about 2.5" while my others are only 2" or so.
Thanks. I've named the big boy Sin, and the blue dragon Rumeri. So woohoo! The koi male flared at me when I approached the QT cup, he's very feisty. My dragon then flared at me next, the big boy seems very relaxed just swimming around quietly. haha.