There isn't really an 'ideal physique'. A lot of it depends on the style of the fighter. I think strong and conditioned legs and back muscles are always a benefit to any fighter though, regardless of style. But some guys benefit from being slighter and others benefit from being bigger and stronger. Look at Foreman's physique; it complemented his style perfectly - strong legs and calves, strong back, and huge arms which facilitated his slower, clubbing but tremendously powerful blows. He could throw arm punches and knock you out such was the strength of his arms. That worked for him. Having big arms would do nothing for someone like Pacquiao or Mayweather or even Ali.
I'd say it makes a huge difference. What do you think would be better in general? Being a 6'7 240 lbs HW with long arms. Or a 5'10 240 HW with short arms and oversized muscles.
Mike Tyson is an exception. How many other under 6'0 heavyweights have been successful in the last 20 years?
Of course usefull muscle mass is important like anyother sport. But the focus shouldn't be the mass but the usefull mass. I mean ofcourse boxers would benefit from lifting heavyweight and getting more explosive. So yes carrying around tons of fat like arreola and John doesn't help them one bit. I'm not saying that the one with the best looking body wins the fight but if a fat guy won over a more well trained guy it wasn't because his fat it was his skill.
Well built core muscles giving an incredible strong set of 6 pack muscles with a nice little beer belly over that as to absorb punches well.
Not to sound like I'm avoiding the question all together, but I don't know if there is an ideal physique as it relates to boxing. However I do think that over muscularity is definitely detrimental to a boxer, especially at heavier weights. I do think body types play a factor in boxing and sports in general, but I think it has more to do with genetics and how an athletes uses his physical attributes.