Nah, it's the cause of her stats. Prana Burst is actually a Phantasm Race skill, it's something that members of the Phantasm Races like Medusa's pegasus have innately. 

[quote]…No, its defense is already at the level of the dragon race. Because that Pegasus has divine protection exceeding even Saber, who has the strongest magic resistance. [b]Its charge, emitting vast amounts of magical energy, is like a giant castle wall approaching.[/b] There is no possible way to dodge or defend against such a thing.[/quote]

Prana Burst is something natural to the Phantasmal Species. Every time they attack, they're using their prana burst skill. So if you don't use that intrinsic skill, you won't get to that level. Saber gets that thanks to the dragon, but without it she's physically weaker than Shirou and Rin.