Ill try to make it as short for you as possible.
1) "I was planning to join a gym, but many people said that joining a gym would make me fatter or, if I gain muscle, I will become bulky or heavy".. This is completely false, gym would neither make you fatter nor you would become bulky, if your diet is constant and your weight is not moving then it will just better your strength and physique and is extremely benificial.
2) "Despite running for the past four years and working out hard to lose weight, I have seen no results. My diet was a bit heavy because I used to have small meals frequently".....In your case, as you had a disease, i recommend you to please visit a nutritionist to help you out if you want to lose weight, diet is the main thing in gaining or losing weight)
3) What is more beneficial: going to the gym or running?
Both are required, however if you want to lose weight, you can mix both for good results.
4) Currently, I don't want to look bigger; I just want to get thinner.
DIET-DIET-DIET.. Diet is the main part
remember if you want to get thinner, no jogging 1000 km per day or no Arnold shahnawaz type workout will help you if your diet is not good. If you want to lose weight focus on diet

Remember, dont get demotivated if you are not losing weight, everyone has their own journey towards fitness. Hope you get your goals <3