system
Given the following post, respond as if you're commenting on Reddit:
human
[Human Body] Are 'fast twitch' muscle fibers anatomically larger in size than 'slow twitch' ones?

In my limited understanding of the human skeleto-muscular system I am aware there are 2 types of muscle fibers. 

Fast twitch: Develop from sprint-like exercise such as running sprints, 50m swimming races, lifting heavy weights close to your maximum limit etc.

Slow twice: Develop from endurance exercise such as long distance running and swimming, lifting light weights for prolonged periods/repetitions.

My question is because in the gymming and weight lifting community there is a consensus that muscle growth (size) occurs when the muscles are put under duress and they fatigue. I am told that fatigue is essential in muscle growth because it generates muscular repair and rebuilds the muscle larger than before. 

Therefore lifting heavy weights is advocated because it puts the muscles under more fatigue and subsequently more growth. Growth of which is that of fast twitch fibers.

But is there any scientific difference in the composition of fast twitch and slow twitch fibers that makes them larger?

If a weight lifter were to lift only <5kg but for prolonged periods of 50-100 reps, that then fatigued the muscle, would the developing slow twitch muscle fibers be smaller than compared to fast twitch ones?

If the answer is no, why do people not commonly build muscle size through slow twitch fibers?

The answer may be obvious but it eludes me. Thank you.
gpt
I actually just graduated with a degree in Kinesiology, so it's good to see it be somewhat useful. The shortest answer is yes, FT muscle cells that have been exercised will be larger than slow twitch. It's the whole idea that human muscle cells grow solely by hypertrophy (growing larger) than hyperplasia (growing in number). However most research shows that mid-high weight (roughly 75%-90% of your 1 rep max) shows the best increases in muscle size over lifting only very heavy weights. ST fibers will grow in size, but not as significantly. If you had someone lifting a 5 lb. weight 50-100 times, then the biggest physiological change will be in number of mitochondria within that cell.