I guess as a Newbie I'm missing something but how is 1500 map segments on the Southeast DVD considered a small amount if I am limited to 3500?
I don't understand the question in the context of what you quoted above. Many older model Garmin models only had 24MB (that's megabytes, not gigabytes) of internal memory and no card slot for expansion. There were also some expensive older devices that only had 8MB. And other expensive models required proprietary garmin storage cards that cost hundreds of dollars just for 64mb.
With an old device like this, small segments were desirable so that you could cover the exact area you wanted. If a segment was, for example, 16MB then you could only load that specific part of the map, wasting 8MB of space in your 24MB memory. People did not think in terms of loading entire regions, let alone the whole US. Maps were originally stored on CD's that you had to insert everytime because hard drives were so small.
I know, sounds like ancient history, but many of these old devices are still out there. Until about a year ago there were still some in Garmin's lineup - even a couple models with serial interfaces (no USB). Garmin still wants to support these, and the DVD map products use small segments for this reason.
If that bothers you, choose the download version next time. I think you will find they have much fewer segments. For example, the DVD version of US Topo 100k has ~6600 segments but the download only has ~450 (like the topo on your Oregon). These are the versions designed for people who want to keep a lot of maps on the GPS. I don't have any specific data on how many sectors the download products feature. You could contact Garmin and ask.
Personally, I don't need all that stuff on my Montana. I keep it lean and mean with just the topo maps I need for my usual travels, plus the download version of City Navigator. I doubt that I have any more than 1000 segments total.
As for what you need or expect, only you can say.