If we accept the Star Wars Christmas Special as canon, we see that Lumpy is an only child. This indicates that litter sizes for Wookies are either small, or that there is an unusually large infant mortality rate for this species.

And if I were to commit the original sin of xenobiology, we could infer that Wookies are primates, possibly great apes, which are not known for large litters.

Finally, raising something as stupid as Lumpy for such a long time means that Wookie offspring require a lot of resources to reach maturity. This is bolstered by the notion that Wookies have a lifespan of 400 standard years, which may indicate a childhood of 20-50 years, further increasing the resources required to raise an offspring to maturity.

However, none of this removes the possibility that Lumpy ate his siblings.

Thank you for coming to my Ted Talk.