What determines lot size of a future

Can you tell me how exchanges determine lot size for a future. I always thought it depends on price. But its not?

see this example
      contract      expiry       price     margin    lot size
 	ALBK      28-JAN-16    51.3      49110      6000
     IBREALEST  28-JAN-16    52.4      83273      9000
Here two future contracts (ALBK, IBREALEST) with same expiry date and similar price (51.3,51.4) have two different margin and lot size. Why?

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