All the the previous responders are right, at least to some extent. One consideration nobody has mentioned is liquidity -- it's a lot easier to move 100 shares than 2000+ shares in your typical Nasdaq roach motel.
I can't see having liquidity problems with 100k, but if he is, then I suppose the market is doing him a great service by letting him know that his method is not scalable and he needs to adapt if he wants to take things to the "next level"...
Because the latter requires a higher percentage return than the former. Liquidity is also an issue. On the other hand, with 100k you have to make a lower % return to meet your living expenses & overheads.
I dont beleive in trading with anything you cannot afford to lose. Thus being said and stndard for both scenarios, this is still true. Pulling the trigger is the hardest.
You forgot the most commonly practiced option: taking $100K and turning into $25K. Some even don't stop at $25K...
I think virtuoso had a good post about buiding your ego from your past success. I also believe it's easier to play on a small scale because you can relate to the money ie 'i just made enough today to buy a hyundai'. When you start getting into 500k in profits it isn't a game of survival, but more a game of success at the highest level. You have to look deep into you motivations and find a solid reason why it's ok for you to make 500k or more. Then believe the reason in your heart it's down hill from there. Scalp100