Precisely true
but there also are other forces at work on inverse % disproportions.
I was curious myself, so I went on a googleplex mission ;)
I found things ranging from swaps, hedges, arbitrage, re-balancing, volatility, liquidity and structural slippage in etf's
I found the most compact explanation from a blogger/poster which seems to make most sense.
The key reason why you observe divergent performance patterns is related mostly to the following:
The biggest reason is the different cost to hedge those products. The costs to implement and especially maintain the hedge on the long vs short side can be very different. Either the hedge is implemented through an index replication in which case the manager has to buy/sell stocks (different costs involved in borrowing shares vs simply buying the shares outright) or through derivatives contracts in which case the manager is subject to opposing effects of being long vs. short contango/normal backwardation, different costs of the roll. So, even though some ETFs may attempt to track an index (many actually track the futures traded on the underlying, such as VXX), what matters for the performance is the hedging costs as well, and if the hedge is done through futures contracts then there is a direct link between the question at hand and my mentioning futures.
Even for simple stock investments the costs associated with long and short positions are vastly different. Long non-margined holdings do not incur any cost. However, short positions incur borrowing cost plus the cost inherent in the risk of borrowed shares being called. Even if both products' underlyings are margined there is a difference between the costs on the long and short side as borrow and loan rates are different.
