There is nothing especially thrilling about trading larger clip sizes, if you get slowly into it and adapt over time.
You cannot just jump from trading 1 contract to 50 contracts, with limited risk capital, and expect to remain calm and controlled in your trading.
But if you do it the right way, and slowly increase your clip size with the profits you make, then the whole process will be pretty steady and nothing spectacular.
Trading plenty of contracts with adequate risk capital does not feel much different than trading just 1 or 2 contracts with adequate (which in that case would be limited) risk capital.
Regarding commissions, of course the more you trade the better position for negotiations with your broker you will have. As a very small retail trader you will typically get commission rates of about 0.80 USD or more per contract.
As a high volume trader who does something like 20,000 contracts or more per month, you either decide to go with a "no commission" or "discounted commission" broker like AMP (2K membership) or Tradovate, or you negotiate with a broker/ FCM that caters high volume traders like for example Advantage Futures. This should bring your average commission per contract below 0.10 USD per contract.
Exchange fees are another thing, but you can reduce those by buying or leasing a seat, or by participating in certain exchange rebate/ incentive programs.
Then you ask about margins... the margin per contract usually does not change with your clip size, but some brokers/ FCMs limit their daytrading margins to a certain maximum amount of contracts.
Instant fills.... this totally depends on the market you trade, and the phase (high or low volatility/volume environment, time of the day...) that market is currently in. In some high volume markets like the FESX, FGBL or ES most of the time I get full fills even when trading with large clip size. Most of the time, but not always. Especially during european lunch time, which I usually dont trade, you might experience partial fills (at least temporary partial), even in the thick contracts like FESX, FGBL and FGBM.
In the the thinner contracts like the FDAX and NQ, I usually see 2 things happen when placing large limit orders on a single price tick of the market: either I get a partial fill for a few contracts, or the market rips through my limit order like crazy, gives me a full fill and moves some more additional ticks immediately.
Because of this, most of the time I prefer to scale into the thinner markets: instead of placing one 20 lot order in the NQ on a single tick, I prefer to spread the order over for example 4-5 smaller orders and place those on different price ticks in the DOM.
All what I wrote here is relating to limit orders, which I use 95% of the time. If you want to know about the fills for market orders, it is even easier: just have a look at the DOM, and you will see the size you can hit and lift within the next few ticks.
