In demo environments, you don't influence the market while with real trades, you do. That's a big difference.
What is influencing the market? Make it move 0,001% or make it move 10%?
And what is a big difference depends on what your average profit per trade is. If you make 0,25$ a share it might be a problem, but if you make 5$ a share the problem is already 20 times smaller.
What will you do if you influence the market and have to get out quickly? Kill your own trade by making the market crash? If your orders influence prices significantly you should trade smaller or trade something else.
In forex the markets size is billions a day. If you buy a few million it will even not be noticed. I bought in past 10-30 million in seconds.
In the ES there are more than 2 million contracts a day. If you buy even one hundred contracts (0,005% of volume) it will not be noticed. Will be filled in seconds.
In many stocks daily volume is so high that buying 1000 shares is not noticed.