What makes a motor hard to spin?

[kazuaki]

I am curious about something I've noticed with my brushless motors. I was used to the stock VXL motors that could be easily spun by hand. When I got my Neu 1112/2D I was surprised at how extremely hard it was to spin. It had the very distinctive 12 notches as it spun and required quite a bit of effort to turn by hand. I figured this is just the way a "high power" motor is. Yesterday, I got my TeknoRC Neu 1512/2D. It is actually very easy to spin by hand. You can still feel the 12 notches in the rotation, but seems much easier to spin than the 1112. Can somebody help me understand what this all means?