It depends on how you get the image into the memory stream. If you read all the bytes from a file into byte array and load that into a stream, then yes the size will be the same. If you load it into a bitmap, then it will depend on the ImageFormat you use when saving to the stream. Take the following example:
using (MemoryStream ms = new MemoryStream(System.IO.File.ReadAllBytes("C:\\temp\\test.jpg")))
{
Console.WriteLine(ms.Length); //59922
}
Bitmap b = new Bitmap("C:\\temp\\test.jpg");
using (MemoryStream ms = new MemoryStream())
{
b.Save(ms, System.Drawing.Imaging.ImageFormat.Bmp);
Console.WriteLine(ms.Length); //6220854
}
using (MemoryStream ms = new MemoryStream())
{
b.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
Console.WriteLine(ms.Length); //59922
}