I also get hit with subscription sales. The royalty is low but it is only for medium sized files, you will from time to time get the odd HD subscription sale but i believe that is for customers using older credits bought before the change came into effect, these will run out in time.
In order to entice you to opt-in to subscription sales they put your portfolio nearer the beginning of the search so that both subscription and non-subscription customers will see your port, and it works. Once i opted out and my sales dropped dramatically. It's not a nice tactic on their behalf, it's either opt in or tough crap.
Fotolia buyers generally are cheapskates, they hardly ever purchase full HD videos, its either non-sub small or medium sized video or sub medium sized video with the odd full priced HD but that's quite rare.
There are things you can do though for damage limitation. For example, only upload your worst clips and then cut them to half the size and encode them down so that they are just passable, i usually have to really compress my videos anyway to get them under the ridiculous 300MB limit. That way people get what they pay for, sounds fair if you ask me.
I would like to upload my best work and make my customers happy but they don't really leave me with a lot of choice.