The reason is clear now, and I think I know how to fix it. When working the increases, I kept increasing well past the point that I needed to stop for my body. This is an obvious realization for most knitters, but there you go.
I now know what a sweater should look like when it's time to separate the sleeves. I'm taking this new found knowledge in my next sweater attempt (even though I have a vest and my Central Park Hoodie on the needles, I know), Pink, by Wendy Bernard. I just got Wendy's book, Custom Knits, and the first design is a basic raglan sweater. I decided this is a good place for me to try again, and hopefully end up with a better fitting sweater that I can wear regularly.
I cast on with one of the first yarns I purchased, Berroco Touche. It's so soft, and the color is really rich. It knits up beautifully. I'm not sure about stripes (the design features a couple of stripes on one sleeve), but I've got three possible yarns to use for that purpose.
Hopefully, I won't lose momentum on this one.