Reason:
INT (default) primary key used by outsystems.
Range: -2,147,483,648 to 2,147,483,647
Storage: 4 Bytes
BIGINT (not supported by outsystems)
Range: -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
Storage: 8 Bytes
DECIMAL To get the equivalent range of BIGINT in decimal, you would need a precision of 19 and a scale of 0. ie DECIMAL(19,0), this will now support the BIGINT range, but with a greater range.
However DECIMAL with a precision greater or equal to 10 uses up 9 bytes of storage, unlike bigint which uses 8 bytes of storage, therefore the difference here is that DECIMAL(19,0) uses 1 extra byte per value stored.
Reference
http://msdn.microsoft.com/en-us/library/ms187745.aspx
http://msdn.microsoft.com/en-us/library/ms187746.aspx