If we're going to be accurate about it, vast sprawling expansionist empires should be highly successful, the best around... for a generation or two. Then they should start getting pounded by rebellion, external alliances against them, leaders who aren't competent enough to maintain what their predecessors built, &c.
The "best" should probably be a middle-ground empire, one that expands enough to have room for a good deal of growth without over-extending, which still has few enough worlds that it can effectively pump the density of a handful of them without totally neglecting everything else.
Basically, "tall" empires tend to face problems dealing with their relative lack of resources; they need to be far more effective at exploiting what they do have just to keep up. "Broad" empires don't really have many problems unless you seriously overextend, stuff like colonizing ten or twenty systems away from friendly territory in a hyperdrive-only campaign to secure key points, and do it in five different directions. "Broad" empires should face problems of consolidation and pacification. They've taken vast swaths of territory, it shouldn't be as easy for them to control it as it is for an empire with five systems to control their shit.
If you go big you should have to deal with big problems. At the moment, you don't. Instead you have to deal with the same little problems more times. If you're familiar with the civil war in the Terran Federation of the Starfire setting, that's an example of the sort of thing big empires should have to deal with: entire sectors of people who're so totally disconnected from the central government ethically, philosophically, and politically that they feel forced to revolt. This shit happens when travel times are limited to those possible on a single planet.