Usually bigger taller and more muscular guys are more stronger than the thinner ones. So does it mean that to get stronger i need to build a bigger muscular body. Is possible to have increase in strength without much muscle gain.if so how to achieve that.
Yes it is possible to increase the strength in your muscles without necessarily getting bigger. It depends on the way you exercise, the methods that you use. Gymnasts usually are very light but possess tremendous amounts of strength. A test for a gymnast's strength is to double his own body weight on all core upper body exercises - Dips, Chin-Ups, Bench press.
Another example you can see on this link. IronMind 1998 Training Hall: Unbelievable Bulgarians - Towards the end of the video you can see Galabin Boevski being 69kgs(~150lbs) squatting 250kgs(~550lbs). Most of the guys in the gyms over here way around 220lbs and are proud when they can squat their own weight. But they train to look big not to gain strength.
Many athletes want to get stronger without getting bigger. It is common in weight-class sports like Olympic weightlifting, judo, mixed martial arts, boxing and wrestling, since the athlete wants to remain the same weight while being as strong and athletic as possible. It's also common in field sports and other competitions such as rowing, where extra weight is expensive to carry around.
These athletes use weight training to get stronger, but not much bigger. Diet is a huge part of this, as is maintaining a high amount of endurance and sport training. In the weight room, they will focus on only a few repetitions of high weight. Singles, doubles, and triples (less than four reps), with a low total volume, will train maximal strength while avoiding a significant degree of muscle gain.