I kind of doubt that he is feeding on other White Court demons, but who knows.
IMO Lord Raith is more powerful because he's enormously older than the others (thousands, vs 100-some for Lara and 40ish for Thomas) -- either because Hunger demons inherently get stronger with age/feeding as groinkick suggests, or because of a 'power split up among descendants' situation such as Odin implies was going on with the Red Court and Lord Raith is from a much earlier generation and thus got a bigger 'slice of the pie'.
Or maybe just vastly more practice allowing him to use his power more efficiently/effectively... I'm not sure the power gap between Lord Raith and Lara or well-fed Thomas is that huge. Thomas has physically matched Black Court vampires (non-masters, admittedly) even when he wasn't feeding much; Lara's Hunger was almost overwhelming Harry with just the edge of its effect, at a significant distance, when she was focusing on Lord Raith at the end of BR. Lara doesn't go all-out "on screen" that often (and the skinwalker in TC probably wouldn't have been vulnerable to even an incredibly powerful Hunger).
Thomas talks about Lord Raith being awe-inspiringly powerful, but he systematically intimidated his descendants; it's entirely plausible to me that in BR Thomas and even Lara thought the power-gap was much larger than it actually was.