Não tenho certeza do que SYSTEM 300 MB é
A partição do sistema contém os arquivos relacionados ao hardware e a pasta Boot que informa ao computador onde procurar iniciar o Windows.
By default, Windows 7 creates a separate system partition when it's installed from scratch (as opposed to upgraded from a previous version of Windows) on an unpartitioned hard drive. This partition is 100 MB in size.
Fonte O que são partições de sistema e partições de inicialização?
Listou apenas 2777 MB como espaço de encolhimento disponível
I'm puzzled, what's causing this and how do I shrink more memory?"**
A resposta está bem na imagem que você postou.
"You cannot shrink a volume beyond the point where any unmovable files are located. See the "defrag" event in the Application log for detailed information about the operation when it has completed."
"See 'Shrink a Basic Volume' in Disk Management help for more information."
Diminuir um volume básico
The shrink operation can be blocked by the presence of certain file types; see Additional considerations for more information.
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Additional considerations
When you shrink a partition, certain files (for example, the paging file or the shadow copy storage area) cannot be automatically relocated and you cannot decrease the allocated space beyond the point where the unmovable files are located. If the shrink operation fails, check the Application Log for Event 259, which will identify the unmovable file. If you know the cluster or clusters associated with the file that is preventing the shrink operation, you can also use the fsutil command at a command prompt (type fsutil volume querycluster /? for usage). When you provide the querycluster parameter, the command output will identify the unmovable file that is preventing the shrink operation from succeeding.
In some cases, you can relocate the file temporarily. For example, if the unmovable file is the paging file, you can use Control Panel to move it to another disk, shrink the volume, and then move the page file back to the disk.
If the number of bad clusters detected by dynamic bad-cluster remapping is too high, you cannot shrink the partition. If this occurs, you should consider moving the data and replacing the disk.
Do not use a block-level copy to transfer the data. This will also copy the bad sector table and the new disk will treat the same sectors as bad even though they are normal.
You can shrink primary partitions and logical drives on raw partitions (those without a file system) or partitions using the NTFS file system.
Fonte Reduza um volume básico