There are multiple protection systems to lower credit and counterparty risk. Margin is just one of these.
This means that the exchange can offer trading leverage as a feature, while still maintaining a low risk system. So you don't need to go to a bank for a loan to get leverage.
Futures contracts are designed to offer attractive trading liquidity and capabilities to institutional investors (both hedgers and speculators) because that's where the big fees can be made.
Most participants are institutions and trade in bulk. This is why the contract sizes are 'large', it makes the trade size numbers smaller (as numbers of contracts) easier to read - seriously. I've actually seen people mis-trade OTC FX deals because 'there were too many zeros and I missed one'. Eg, 1000000000000 KRW - is that ~1bn USD or ~100m USD? etc
There is another answer here, which is to do with encoding numbers in computer systems. The upper size limit for a signed 32 bit integer in most computer systems is a little over 2.1bn. However, that is a small number of dollars, Yen, euros etc. To improve speed in computer systems, numbers like number of contracts in a trade is kept under ~2bn so that they fit in a 32 bit integer, by making each contract worth 10k USD or more etc.