I don't think either of the explosions were related to the rockets getting bigger or more powerful. The CRS-7 failure would have happened on any version of the Falcon 9. It happened on that particular one because that one happened to have a defective strut. The AMOS-6 explosion happened because of their experiments with supercooled propellants.
Two losses in two years is a lot better than the very early days when they lost their first three Falcon 1s in a row, but worse than the early years of Falcon 9 where the only failure was a single engine out that didn't hurt the primary payload, and is certainly not a good track record overall.
Two losses in two years is a lot better than the very early days when they lost their first three Falcon 1s in a row, but worse than the early years of Falcon 9 where the only failure was a single engine out that didn't hurt the primary payload, and is certainly not a good track record overall.