Answering “why x firm?” For a small boutique

Usually it's some mix of:

• You like that it's a smaller team so you have exposure to the senior bankers (less bureaucracy in the middle) and can take on more responsibilities as presumably deal teams are smaller
• You like the idea of working with smaller businesses that will require more hands-on engagement by a banker (since many will be run by entrepreneurs or families that won't necessarily have had a ton of experience in M&A situations)
• If this firm has a specific attachment to a geography you have ties to, then you can play that up as well (desire to be there longer-term)
• Do some research into this boutique online...do they have a specific niche or area they focus more on?  If you have a legitimate interest there, then play that up as well.