There are two main methods for conversion.
The first is the regular assimilation ability, which works by having the target take damage during a brief period of time while it's being debuffed. A large fleet of Miranda's will have nearly twice as many ships as a fleet of Excelsiors does. The math is pretty easy here, twice as many targets to fire on means a much lower chance of being damaged at the right time to be converted. It's a case where bigger is most definitely not better.
The second is the capital version of the cutting beam, which converts any ship that is held for a sufficient period of time after it's shields are down, a significantly easier accomplishment against the Miranda thanks to the shield deficit. Bigger is inarguably and insurmountably better in this case, as not losing Miranda's to this ability is all but impossible while an Excelsior actually has the shields to do combat against a low level Queen Diamond, if not for long.
The regular assimilation method greatly outweighs in terms of overall impact. What it lacks is the ability to steal the big dogs from the capital ship slots. A mixture will always be better than using single ship fleets, high point, durable ships need to be screened with smaller ships or they'll be too vulnerable to drains and conversions.
