but the database instance contain 2 databases each with size 240 GB and 714 GB respectively. Dont you think that 80 percent memory is very less.
No I don't think so and as I said previously, monitor the server and adjust if necessary. Two things to take into consideration, firstly if the OS is pressured for memory i.e. paging/swapping memory constantly then the SQL Server is going to feel that pressure as the OS will be reclaiming memory back from it. Secondly the sizes of the databases are almost irrelevant here, more useful information would be number of concurrent users/connections, transactions p. second, type and volume of activity etc.