The effect would not be very noticeable unless the large planet was extremely large / high gravity (e.g. it would look "almost" normal) looking at one another. If the planet was that large, it would probably not be able to support life, but to see what would happen, lets assume it can support life.
Relativistic case, from the large planet, observing the small planet:
If the large planet were extremely "heavy / massive" (think more like a neutron star), the people on the large planet would observe the people on the small planet moving much faster. It would be like watching a movie with "fast forward" on. However, light would also be blue shifted, which would produce other interesting visual effects (e.g. what was red might now be blue, and what was in invisible "infrared" might now be red depending on the amount of the shift.) The amount of blue shift (light wave compression) and "fast forwarding" would increase if the planet were made bigger. The rest of the universe would have similar effects when observed from the large planet.
Relativistic case, from the smaller planet, observing the large planet:
The people on the smaller planet would observe the people on the extremely large planet moving in "slow motion". It would be like watching a movie in slow motion. The people on the large planet would also be "red shifted". For example, something that was blue might appear red, and something in the ultraviolet range might now appear blue. The "slow motion" effect and red shift (light wave expansion) would increase if the extremely large planet were made bigger.