In addition Servos have a big overhead for short periods, usually 2 to 3 times the constant torque.
For a Bridgeport sized machine I would think 750Watt AC servos would be more than adequate at 2:1 reduction, some people reckon even 400W would suffice although I am not so sure about that, suppose it depends what you will be doing and cutting and what accel and Vel you are wanting.
Hood
These days it is difficult to compare machines by 'size' so I suggest focusing on the weight (mass) of the load and not the size. For example, I just finished a servo conversion on the largest asian bench mill, namely the Industrial Hobbies mill with the 'oversize' table. While the table is close in length and width to the a bridgeport table, it is only half the weight. Another difference betewwn servos and steppers is that you can oversize a stepper without negative consequences. Oversizing a servo can result in all sorts of problems including making the system un-tunable, defeating built in auto tuining and/or having the motors break into uncontrolled self exciting harmonics (given an elastic connection to the load).
The IH conversion has 400 watt motors moving the 150lb table. An actual Bridgeport table is over 300lbs and, as you suggested, would require the 750watt motor to achieve similar performance.
On the IH conversion, the head was scratch built and weighed in at over 300lbs so the Z axis in fact has a 750 watt motor.
Mitsubishi publishes a software specifically for sizing their AC servo motors and that is what I use to spec the motor size for a particular application. It is a bit complicated and requires a lot of technical details about the application including things like the coeficient of friction of the sliding members and stuff like that, but one could also use a 'rule of thumb' approach and probably not get into too much trouble. If one accepts that 400 watt is good for a 150lb table (with dovetails) and 750 watt is good for a 300 lb load (on ball slides), then it might be reasonable to extrapolate a rule of thumb like 1.25 to 1.35 watts per lb. of total mass to be moved (for a machine tool application).
Note that for calculating the horizontal movements of the table, the weight of the table (but not the mass) can be discounted. A 'live load' however shoudl be included since this is the actual typical operating condition. You might consider 1x the table weight as a reasonable WAG. So in our examples, the IH table would carry a 150lb load for a total mass of 300lbs to be moved. 400 watts / 300lbs = 1.33 watts per lb. Similarly the 300lbs bridgeport table carries a 300 lbs live load (mill vice, 4th axis, fixtures, workpiece, etc) for a toal weight of 600lbs. Therefor 750watts /600lbs = 1.25 watts per lb.
The Z axis is a bit different in that ther is no live load . . other than the head itself . . which has 'weight' to be lifted in addition to the mass to be accellerated. So if the head is fully counterbalanced (air springs, aka struts, or equiv) Then just use the all up weight of the head as the load to be moved. If the head is not cojnterbalanced, then add the 'weight' of the head to the mass of the head and use that.
DISCLAIMER;
the proper calculation method for motor sizing are complicated and the 'rule of thunb' described above is not presented as the correct nor the preferred, nor accurate method to calculate servo motor size. It is a theoretical context without warranty express or implied . . . use at your own risk . . . batteries not included.