You cannot, dynatrace is allocating available memory automatically (with reservation for each components). You have to know that there is not only Java server, but elasticsearch, cassandra nginx etc as well. All of those components has to have resources available. This is why it is possible that dynatrace is not using all available memory on host.
In addition to Sebastian comment (which is also the answer from Dynatrace people), if you look at the content of the Dynatrace Managed installer you will also found this (which I BELIEVE is the percentage of memory allocation):