I don't buy the "strength was my own" excuse, either.
One thing that changed in Bonds after he bulked up it that during that same year, the rate of the shoulder turn in his swing went from something like 24 rpm to 30 rpm (I'm trying to find the link, but I must have about a thousand in my "Baseball" bookmarks folder). With that kind of super-human shoulder turn, it is no wonder his contact rate skyrocketed and sweetened - he could turn on any pitch. Without the roids, there is no doubt that some of those HR would have been swings and misses or, at a minimum, less sweet contact that would not have resulted in a HR. I haven't seen any similar comparison of the skinny McGwire vs the bulked McGwire, but I expect that he received a similar benefit.
[EDIT: I haven't found the article I'm looking for, but did find this analysis of roided Bond's swing by Tony Gwynn. Note, especially in frames 5 and 6, how he is able to turn on the ball without changing the relative position of his arms. You may note earlier that Gwynn references that is it impossible to pull the bat through without using your arms and provide any pop, then notes that Bonds can do so (thanks to his 25% increase in shoulder rpm).Gwynn's analysis of Bond's swing