I assume that by "riding fixie" you also mean "riding brakeless", or at least trying to master stopping without using brakes, only using your legs and modulating your weight. Changing the gearing is also a way to make it easier or possible, as I will try to show below.
To make any wheel skip, the friction between a tire and ground must exceed a critical value for friction force Fgliding. Below that value, friction force is defined by equation F = µN, where µ is a constant coefficient depending on the surfaces in contact, and N is reaction force, which is equal to the weight loading that wheel.
Under the drivetrain braking, the deceleration force (== tire/ground friction) will be equal to the force your legs apply to pedals multiplied to a constant. This constant is defined by your front and rear teeth count, cranks length and wheel radius. Basically your drivetrain is a lever, to which you apply one force at one end and get a transformed force value at another end. You cannot load your pedals more than you weight (more or less). Thus, your legs and body can create a limited amount of equivalent stopping force at the rear wheel, let's say Flegs.
If Flegs < Fgliding, you will be unable to skid. You'll still be decelerating, but the rear wheel will be turning while rolling (not sliding) over the ground. Because it is a fixie, pedals will also be turning "against your will", forcing your legs to obey their rotation.
To make it skid, you'll need to either 1) increase Flegs, or 2) decrease Fgliding. To affect the former value, gain weight or change your gearing combination. To affect the latter, you can either reduce µ, which is exactly what happens in the rain, or reduce N by unloading your rear wheel.
To illustrate it a bit differently, if you throw your rear wheel just high enough so that it does not contact the ground at all (N= 0), your will be easily able to stop its rotation with legs while it is in the air: a wheel's inertia is obviously less than that of the whole bicycle + yourself. When the wheel lands back to earth (N > 0), it will be stationary (relatively to the frame), while the ground at the contact point will be moving relatively to it (because the whole bike has not yet stopped). This non-zero relative speed between the ground and the wheel's contact point is the definition of skidding: instead of rolling, the wheel slides. The re-obtained non-zero friction will cause the wheel to gradually start gaining its rotational momentum again; but that cannot happen instantaneously. It will continue skidding until the speed of its contact point becomes zero, at which point it has again regained its full rolling contact and speed.
This is exactly what happens when braking a mountain bike over an uneven terrain. The bike "flies" for short moments from root to root, stone to stone. Wheels' rotation, while in air, is halted/slowed down by applied brakes. Upon landing wheels produce this "skrrt-skrrt" sound of spinning back while skidding for short periods.
Given that there is a small negative gap between values of rolling and sliding frictions, this state of the wheel gliding can be made "stable", not temporary. If you find a sweet spot where the rear wheel is loaded just enough so that Flegs = µN, you will be able to skid continuously until a full stop.
Sheldon Brown describes the same physics behind the process, while giving a bit of details on how to place your body and pedals to unweight the wheel: https://www.sheldonbrown.com/fixed.html#skip . Pay special attention to health warnings he gives about doing it on that page: "Heavy-duty resisting is widely reputed to be bad for your legs, and to be counterproductive for building up muscles and coordination for forward pedaling."
