Es lo que propone un post de Morningstar (que casualmente provee reportes sobre ETFs, hay ¿PR?). Más alla de ello tiene puntos que valen ser reproducidos, como el siguiente argumento de escala
While the ETF is quickly replacing the mutual fund for stock indexes, the same is not true for bond indexes. While the stock market is highly liquid and stock prices almost never go stale, this is not true in fixed-income markets. Bonds often trade infrequently, and transactions still are largely conducted over the telephone. Perhaps more importantly, the asymmetric return pattern on bonds demands proper diversification. With bonds, the maximum upside we can expect is capped at maturity as a bond will pay only par plus the coupon. But if the bond defaults, the downside can be 100%. Thus bonds returns exhibit a negative skew. Just one default could cause a bond portfolio to underperform. Whereas active stock fund managers prefer more nimble, smaller portfolios, bond fund managers require the economies of scale of larger funds. Thus smaller bond ETFs may not be able to reach the scale necessary to achieve diversification.