For some people, the goal at the gym is not to become big and bulky, but simply to increase strength. There are many sporting and fitness disciplines that require you to be very strong, while also requiring you to stay lean (e.g. gymnasts, sprinters and lightweight boxers). With this in mind, the challenge at the gym will be to increase your strength and capacity while keeping your frame tight and slender.
You don’t have to change your routine to achieve this, but you do have to alter your weight distribution. Many athletes, as earlier mentioned, increase their strength by lifting heavier weights than normal, and doing their usual exercises as quickly as possible. Lifting heavier weights will engage the muscle fibres that build strength in your body.
However, you will only be able to maintain this for a short period of time as these muscle fibres tire quite quickly. It is in your best interest to be doing exercises that engage more than one muscle group (e.g. weighted pull-ups or deadlifts) so that when the fatigue kicks in, you will have maximised your workout.