Proportion of heads and tails over time
When you flip a coin 10 times, it's not that unlikely to flip 7 heads and 3 tails. But if you flipped the coin let's say 100 times, it would be very unlikey if you flipped 70 heads and only 30 tails. This is because the proportion of heads/tails is fluctuating a lot when you flip the coin only a few times. It starts to approach 50/50 if you flip the coin many times.
To illustrate this point, I conducted ten experiments where in each experiment I flipped the coin 300 times (not an actual coin of course, a computer simulated one, I have better things to do than flip a coin 300 times). After a couple of flips, the heads/tail ratio fluctuates a lot. Even after a 100 flips the heads/tail percentage is somewhere between 40. and 60%.