Minecraft refuses to use more than 1G of RAM, even via Command Prompt

Allocating more RAM to Minecraft can do two things:

1. If the game's performance is being limited by a lack of available RAM, it will ease that possible performance bottleneck.

2. If the game is crashing due to Java running out of RAM (either due to bugs or due to complex constructions/mods in a world), it will postpone or completely prevent the crash from happening.

Whether allocating more RAM does either (or both) of these depends highly on what the source of the performance problem is in the first place; your particular computer's configuration and components; the size of the texture's you're using (32×32 takes 4× the RAM the default textures do, 64×64 takes 16× the default, etc.); the details of your Minecraft world(s); and what mods (if any) you have installed.

The default amount of RAM set by the Minecraft launcher is 1GB. This is usually plenty, but spikes in RAM usage or unusual circumstances can make it inadequate. If the game is normally running OK on 1GB but it's having a few issues sometimes, doubling it to 2GB should be plenty. If doubling it doesn't help, then the problem may not be solvable simply by giving the game more RAM and you may need to address the problem at the source: removing demanding mods, reducing texture pack size, solving physical heat problems, upgrading system drivers, turning off background processes, making redstone contraptions more efficient, etc.

Looking at the JVM arguments you are using I don't think there would be a problem. As I stated in my comment, the Xmx argument is a maximum setting and will only ever be reached if Minecraft has enough chunks loaded, so to put it simply having a higher Xmx value will allow to have a higher render distance set in game (this is an extreme over simplification).

That being said there is an additional option you can specify that will allow you to tell the JVM to allocate that memory immediately, rather than as needed; the option looks like -Xms2G. However this will only provide a very minimal performance increase if any at all.

I would also try removing the Xmn argument, this setting is used for things in memory that are short lived and it will likely be cutting into the 2G overall max (don't quote me on that I could be wrong)

I am not too knowledgeable on the other JVM arguments you have specified but I did find this article that mentions to avoid using both UseAdaptiveSizePolicy and CMSIncrementalMode and that UseConcMarkSweepGc and UseAdaptiveSizePolicy are contradictory settings

A quick final note, if you are trying to get Minecraft to use more ram to improve performance, it is much more likely that your processor or video card are performance bottlenecks here than your ram. I especially think this because you are already providing it with more memory than it is using.