Why are shirt sleeves so small?

I'm starting to get tired of shirt sleeves on about 90% of my shirts (including Red Cross, and technology shirts) fit tightly around my arms and chest, so I was wondering, why the shirt sleeves are designed the way they are?

To give a little background to imagine what I look like (pictures on previous posts):

• someone at the food shelf told me I look like "a unit"
• 6'3"
• 240lbs
• best bench press 200 lbs (cables per arm)
• best curl 97.5KG+45 LBS (cables) per arm
• throw around boxes of bananas 3 high (USA) normally at the local food shelf
• train with 90 LBS on my neck (cables on a loop on my forehead)
• for the country music fans my name is Big John (kidding)