You can increase the mempool size, if you have an interest in seeing transactions that likely won't confirm for a very long time, but you certainly don't have to. The mempool is sorted by feerate, and when the limiter kicks in, it removes the lowest feerate transactions. Since miners sort by feerate, and can include only a limited number of transactions per block, the mempool effectively works as a queue.
By design, you will not always be able to store all unconfirmed transactions. If your goal would be to have all, and crank up your memory pool to accomodate this, someone could just start creating an infinite stream of transactions sending back and forth, until your node goes out of memory. This is the reason why the mempool size is limited in the first place: protection against a denial of service on the network.
Please include the relevant information in your post, link only answers are considered low-quality and may be removed. If you found the information on another page, adding a link to the source is appreciated of course. – Murch – 2016-10-27T15:50:38.460