It can be a really huge file because it contains the entire contents of your system memory dumped into one file, just in case there is something important that could have been lost when your system crashed.
The memory.dmp file is not always present on a computer, however if it is there, it will be taking up alot of disk space, and you can delete it after you have rebooted.
I have seen it on my computer several times and it is always taking up in excess of 100 megabytes of space, and this is not even set to maximum size. When I delete it there is a noticeable jump in available space.
The memory.dmp file can be huge, ten times as big as what I have seen - up to 1,000 megabytes or in other words 1 gigabyte. It depends on whether it is set to Complete Memory Dump as well as how much memory you have installed on your PC.
Where will you find it? That depends on the operating system you are using. In Windows XP you will find it in the C:\WINDOWS folder. In older operating systems like Windows NT or Windows 2000 you will find it in the C:\WINNT folder.
If you would like to, you can change the Windows settings so that the memory.dmp file is not so big next time your system crashes. This is called performing a Small Memory Dump rather than a Kernel Dump or a Complete Dump.
To do this in XP you have to do the following :
Now your computer is set up to record only the very minimum of memory if a crash occurs, just enough to help identify the problem that caused the crash. The file created now when your system crashes is called minidump as opposed to memory.dmp, and it is located in C:\WINDOWS\Minidump
If you have selected the Complete or the Kernel options in the dropdown menu, then to avoid more disk space being taken up each time your system crashes, check the "Overwrite any existing file" checkbox. This means a new memory.dmp file is NOT created every time. Instead the old file is overwritten.
By the way, the memory.dmp file isn't usually defragmented. That is because it is a paging file, or in other words a Windows system file. It is generally set to low priority for defragmentation.