Yes, this is also what I am seeing (my Mac Basecamp is old, version 3.3.2, but it seems to work the same). My map was intended to be used with detail set to at least the middle position ("medium" on the Windows version). When set like this, all the minor features will disappear when you zoom out to .7 miles and the whole map will disappear after 5 miles. Try increasing the detail some more and you will find the "sweet spot".
When you zoom out further than 3 miles, it's going to look like a mess no matter what you do. It just was not built in such a way as to work well when zoomed out farther than that; the roads get too fat in relation to the other features. For my own personal use, I never zoom out farther than 1.2 miles with this map. That is just getting beyond the optimal scale to view this amount of detail.
On the GPS, try setting map detail to either "more" or "normal". This will make the minor features disappear at either .8 or 1.2 miles, giving a cleaner view of the map as you zoom out.
Keep in mind that this map has an extreme level of detail due to the landcover and high resolution contours, maybe 10x greater than a Garmin map. This makes it behave a little differently. I think Garmin's older windows software "Mapsource" handles it a little better for some reason.