I cannot afford a gym membership or expensive workout equipment, so I do isometric exercises. I heard that by doing this type of exercise, you could actually increase the size of your muscles without lifting or curling anything.
I found this to be true. I would push against a wall with all my weight for 20 seconds at a time four times in a row. Also, I pressed the palms of my hands together with the force of my arms for 40 seconds four times in a row. After a short while, I noticed that my biceps and chest muscles did appear larger.