Small vessels cannot hold light/very small items anymore.
Seeds, feathers, food do not go in.
medium/normal things like nuggets work
light/small things like straw works
medium/very small things like thatch work.

Meta Info

• TFC Version: build 92

Sorry, I poorly fixed the last issue. Looks like what needs to be done here:

IItemSize cap = CapabilityItemSize.getIItemSize(stack)
if(cap != null)
{
    ...
}

@alcatrazEscapee

Ah, so tfc seeds don't implement IItemSize either, assuming they'd get light/very small by default?

TFC items generally implement IItemSize as an interface, as it's simpler.
TFC blocks implement the interface, but require ((ItemBlock) stack.getItem()) instanceof IItemSize to access the interface
Vanilla and other mod items add it as a capability, as we can't implement any interfaces on existing blocks or items.
Thus the reason we have a helper method to get the size from a stack instead of doing instanceof directly.