It's a joke about mass vs weight, since the imperial "pound" is unspecific as to being pound-_mass_ or pound-_force_, the latter being based on Earth's standard gravitional field.
The conversion of kilograms, mass, to pound-force, weight, relies explicitly on the given acceleration of Earth's gravity, ~9.8 m/s².
Since Mars has a gravity with acceleration ~3.7 m/s², for two things that weight the same on Earth and Mars the latter would need to be larger in mass to weigh the same in pound-force.
The biggest issue though, and something I will say even having been through multiple physics courses myself I don't recall having had literally explained, is that even though "pounds" can refer to force and not mass, they are not equivalent. 1 pound-mass ≠ 1 pound-force.
It is actually converted using the Earth's standard gravitional field's force, so 1 pound-mass ≈ 9.8 pound-force.
Thus, the amount on bench's is the pound-mass amount, say 66 kg or 145 pounds-mass, which is not 145 pounds-force but actually *1421*.
It's also true even that the gravitional force can change up to as much as 0.5% depending on where on Earth you are due to the shape of the Planet.
While the joke made _me_ laugh, but it seems as you dig further there isn't often reference to pounds as a force as much as I had thought, so I even learned something from it too!
Lots more information on Wikipedia[0] around these that is particularly useful if you for any reason want to know more:
0: https://en.m.wikipedia.org/wiki/Pound_(force)