How can I thicken up my scrawny String of Pearls plant?

Never leave plants in plastic pots, roots rots easy, use clay-pots. Take plant out, check the roots, remove bad roots, put plant in a clay pot. A good idea to give yearly fresh dirt. Good luck!

Trim it back, put the trimmings in water to grow roots, and then give it more light. How is the soil? Water deposits? Mold? If so, dump the soil outside and replace the soil. Fertilize it with something like Miracle Grow in distilled water.

It looks really healthy ; they do grow long,spindly & trailing you may want to just buy another/more and add to it to thicken it up.Pinch the very tips off so it will grow from base of plant.Any pearls that fall off?? place the pearls in/on dirt they will start new plants along with any decent size pieces you pinch off.If you add more don't increase pot size keep in smaller pot more rootbound,too big of pot promotes root growth not growth on top of plant,give it some miracle grow water soluble plant fertilizer every 6 mos., really bright light(your looks good)#1 killer of house plants is overwatering.

and I would put it in a way smaller pot, so that it will get root bound. Once it gets root bound then move up 1 size in pot, with new soil and drainage, fertilize. Give it Miracle Grow once a month to get it growing.

Yes you are right A lot of certain plants if put in to big of pot it will wait until the roots fill in before the tops grow up and out. I found this out with my plants I have grown. Certain house plants can be tricky to grow so you need to learn what each plant requires.

String of Pearls likes bright light, but grows along the ground where it is shaded by other plants. A bright window is a great place to put it, but your plant is hanging too high and may not be getting enough light. It needs well drained soil, and cactus mix is recommended. Don't water more than once a week. Soak the soil then let it dry completely before you water again. This plant prefers dryer conditions during winter so don't water more than once every two weeks. Cooler temperature, around 60ºF, are also advisable for the winter. If your plant is healthy a dry, cool winter will induce it to bloom in the spring. The pearls of this plant are somewhat toxic if eaten, so keep them out of reach of pets and young children. Here is a link on reasons your plant may not be doing well. https://www.joyusgarden.com/10-problems-growing-a-string-of-pearls-plant-indoors/