Clarification using _MEM

[TerryRitchie]

In the wiki under _MEM is this:

TYPE memory_type
  OFFSET AS _OFFSET      'start location of block(changes with byte position)
  SIZE AS _OFFSET        'size of block remaining at offset(changes with position)
  TYPE AS _OFFSET        'type description of variable used(never changes)
  ELEMENTSIZE AS _OFFSET  'byte size of values inside the block(never changes)
  IMAGE AS LONG          'the image handle used when _MEMIMAGE(handle) is used
  SOUND AS LONG          'the sound handle used when _MEMSOUND(handle) is used
END TYPE

I'm interested in using SIZE to determine how much of the memory block is left to process, if that's possible.

I'm getting an image's memory location:

MemBlock = _MEMIMAGE(ImageHandle)

Then getting its offset:

Offset = MemBlock.OFFSET

Then iterating through the memory block:

DO
    _MEMGET MemBlock, Offset, Pixel
    'other code here
    _MEMPUT Memblock, Offset, Pixel
    Offset = Offset + 4
LOOP UNTIL MemBlock.SIZE = 4 ' (or perhaps 0?)

However MemBlock.SIZE never changes in value as I navigate through the memory block. What does the wiki mean by stating " size of block remaining at offset(changes with position) "?

Any help would be appreciated. I'm delving into the MEM statements and making great progress but this has me baffled.