Increasing the available memory only has an effect if your PC has this amount of RAM. It only defines how much of your RAM can be used by Java (SNAP in this case).
For example, if you have 8 GB of RAM, increasing the max value to 16 GB has no effect.
m in this context stands or megabytes and the values should ideally be a multiple of 512 (explanation)
You can more flexibly also use G instead: 2G are 2048m, 4G are 4096m, and so on…
So if you have 16 GB of RAM, you can set it to -Xmx12G
If you only have 8 GB of RAM, you can set it to -Xmx6G