Ben-

The concept of "scale" is actually more appropriate to paper maps or electronic maps that actually started as paper maps.  For example, when you zoom a Memory Maps image, you actually started out with a digitized version of another map (at some scale).  Thus, when you "over zoom" the image begins to pixelate (get blothy).  These kinds of images are referred to as pixel-based.

Topo Canada (and other MapSource) maps are digital maps from the "get-go."  This means, they are essentially re-rendered at each zoom level (and with this particular product, you can add or eliminate your desired level of detail shown, e.g., frequency of topo lines).  You can still "over zoom" these maps because the digital image is only detailed to a certain level.  So, even though you can zoom down to a scale of 80 ft per inch, the edges will be somewhat "ragged."  I generally work at a level somewhere around .2 mile per inch, at which level the details are still rendered relatively smoothly.  These types of images are referred to as vector-based ... the map is actually a mathematical representation rather than a "picture."  This is the principle reason that you'll notice a bit of a lag on slower computers between when you ask for a particiular zoom level and when it is presented on the screen (or a bit of a lag as you scroll to areas adjacent to the screen you are looking at.)

Does this help?

dd