Don't lose any sleep over it...I'd have done the same thing, & snakes in the wild will too.You just want the sum total of the smaller ones to be approximately the size
you'd have fed with only one prey item...just common sense. And actually, some snakes digest multiple small items better than one large item, but the reason we go with
adult size rodents is that there is more bone mass & hence, more mineral nutrients, & young rodents are usually a higher fat content as well, while adults are higher protein (muscle).