At that point it wouldn't make sense to have it as a hand held weapon, and instead it would just be a tank, artillery platform, or a weapons satellite like the Bradley-Hercules The Enclave used.
If it had the effects the bomb on megaton has, which doesn't make sense to begin with, given its so much smaller, you would have essentially just remake the Dacy Crockett nuclear device, which was never used partially due to how completely idiotic putting that large of an explosive device on a hand held weapon is. It would be pure suicide to use it in almost any given situation.
With that large of explosive blast, it could only be used from one place in the game, aka Tenpenny Tower, and only against certain target, and then the other 99% of the game it would be utterly useless, even if you had ammo for it, since using it would kill you. Thus turning it into a joke weapon since you can't actually really use it in normal gameplay outside one highly specific situation.
The only nukes in Fallout 1/2 were also many timers larger then the mini-nuke, on par with the megaton bomb.
And even then, The nuke under the Cathedral in Fallout 1 didn't do any significant damage to the city of LA itself, only that one building. Fallout 3 actually grealty increased the explosive power of nukes, by having it wipe out all of Megaton, which was several times larger then the Cathedral.