When i was messing around with the accumulator it showed me this:

Also i saw that the 2x2x1 and 3x3x1 accumulator had the same capacity as a small one:

Oh and sometimes when i placed the 3x3x1 it was half full (i didn't copy any nbt) Sorry, i didn't take a photo of it :(

Additional info:

• Mod version: 1.2.3
• Forge version: 43.3.2
• Create version: 0.5.1f

UPDATE:

Accumulators with a minus capacity can't store energy.