I'll assume that your world is spherical and not rotating fast enough for centrifugal force to make a difference. The surface gravity then depends on two things: The planet's mass and its radius. Let M be the planet's mass in units of the Earth's mass (i.e., Earth has mass 1.0.) Let R be the planet's radius in terms of the Earth's radius (i.e., we're using units of about 4000 miles for radius.) So Earth has M=1.0 and R=1.0.
For another planet, the surface gravity would be M/R2 (in units of the Earth's surface gravity.)
So for a planet twice the mass of Earth, but the same size, the surface gravity would be 2.0/1.02 or 2Gs. For a planet the same mass as the Earth, but twice the radius, the surface gravity would be 1.0/2.02 of 0.25Gs. And so forth.
Looking at your two specific questions, you are asking about two planets, one 90% smaller than Earth, and one 25% bigger. H have to know the planet's mass, so I'll assume that your planet has the same density as Earth (it's reasonably close to correct and much easier to figure out.)
Begin with the formula for surface gravity in terms of the mass and radius of a planet, M/R2. Assuming that all planets have the same density as Earth, the mass, M, would be R3 since a sphere's volume scales as the cube of the radius and the mass of a constant-density sphere is proportional to its volume. (We're still dealing with M and R measured in terms of the Earth's mass and radius.)
Plug this into M/R2 and we get R3/R2 which reduces to simply R. In other words, the surface gravity of a planet the same density as Earth is simply proportional to its size.
So surface gravity for your planets, one 90% smaller than Earth, and one 25% bigger, would be 10% of Earth's surface gravity and 125% and people would weight 10% as much and 1.25 times as much.
(As an addendum, it's worth noting that the assumption of all these planets having the same density is not bad, but it is not exact, either. First of all, it applies only to rocky planets. Earth's density is around 5.5 (in the most common units). In those same units, a gas giant like Jupiter has a density of only 1.3, so the constant density formula would drastically overestimate its surface gravity. The Moon is rocky, but because it's less massive (and also because it contains less iron) its density is only 3.3. As a good rule of thumb for rocky planets, the more massive a planet is, the denser it is because the planet's own gravity compresses the rock somewhat. So the simple result that the surface gravity is proportional to R will tend to underestimate the gravity of larger planets by a bit and overestimate the gravity of smaller ones by a bit, also. But not by a huge amount.)