When you increase your lot size, you stand to make bigger profits (and losses too) but the margin required to open the trade also becomes higher. So, with a small account balance, you will not be allowed to trade big lot sizes.
If you are not an expert trader who has been making profits consistently, avoid big lot sizes cos you will blow your account in seconds. A friend of mine blew a $1,000 dollar account in a few minutes of trading big lot sizes.
For me, I stick to my 0.05 lot size for peace of mind, if i see the trade is going in my favor i may add more 0.05 lots to it gradually.