Straight nuclear weapons in space are almost a waste of time. The energy release is in the form of a spherical emission of x-rays, which cannot be converted into other forms of energy in a vacuum. The massive energy release of a nuclear weapon on earth is from the x-rays impacting the atmosphere and converting it into a hypersonic plasma shockwave.
The other issue in space (or even on Earth) is the energy release is spherical, so the "inverse square" law takes effect:
Quick reference to the inverse square law
What is needed is either to land the weapon for a ground burst (to transmit the shockwave through the ground to collapse the colony), or even using a penetrator to allow the bomb to explode under the surface for even greater effect.
Using the weapon as a energy source to "drive" effects is a much better use of the energy of the weapon. Using the bomb to drive a monster HEAT or EFP warhead allows the weapon to be orientated and aimed in space and fire from a distance. The energy is focused and a metal slab can be used to deliver this energy deep into the target.
Velocity diagram of a HEAT charge being formed
Since we are now talking about accelerating metal slabs or streams of liquid metal at 3% of the speed of light, small nuclear devices can deliver a massive blow against even a deeply buried target.
The solution is the spaced NEFP. Its effective range is practically infinite. A 1 megaton warhead could propel a 2.7 ton projectile to 800km/s, while massing only about 3 tons. This projectile crosses the laser's effective range in about 30 seconds, gouges out a crater nearly a 100 meters deep and/or cracks the target in half with 2160 kN.m of momentum concentrated on a spot less than a meter wide.
It isn't clear that a 6Mt warhead will radically improve this sort of performance. Obviously a 6Mt warhead can drive a much larger plate and deliver more energy to the target, but it might be more efficient to send 6 1Mt warheads in a hexagonal pattern and deliver strikes which can collapse even deeply buried structures while also being dispersed across enough area to destroy separated structures.
Indeed, using nuclear devices as energy sources allows you to create a cloud of much smaller warheads, and use this energy to destroy surface mounted sensors and weapons systems in advance of several larger devices delivering EFP strikes against identified enemy strong points. Nuclear weapons can drive clouds of pellets at 100 Km/sec, star hot spindles of plasma at a fraction of the speed of light or streams of liquid metal at 3% of the speed of light.
Cross section of an ORION nuclear pulse unit. A nuclear shaped charge weapon would resemble this device
This allows you to target individual structures and ensure maximum damage to those targeted. You can also use this refined targeting to ensure the main living quarters are untouched to reduce collateral damage and provide a great propaganda victory for the attackers.
Edit to add:
One of the comments asked why "only" a 6Mt weapon. There is no actual upper limit to the size of a nuclear device. If we want to go "big" (and quite the opposite to my answer), the largest weapon I have ever heard described in a semi-plausible way is a 5000 Mt device. For kicks, it was required to reach Soviet missile fields in 2 minutes, meaning the massive 460,000kg warhead (at 11Kt/kg) would need to be boosted by an ORION nuclear pulse drive...neofuel.com/inhabit/inhabit.htm#cheek_on_bomb. See also worldbuilding.stackexchange.com/questions/60557/…