Singles: Steps were around in the era of mega single sales. Kylie's PWL era was was a muich lower selling era overall, I would have thoughht.
Albums: "Step One" I imagine won't be far behind Kylie, Steptacular probably sold more than Enjoy Yourself (it was a million and a half ish if I remember), Buzz would have outsold Rhythm of Love and, yes, Steps Gold was another million seller if I remember.
Internationally, as well, Kylie wasn't actually on PWL everywhere. They had loads of random distribution deals outside the UK. So whether you count that or not is arguable too.
I've just Googled it and the sophomore albums from both acts sold roughly the same amount.
However the third Steps album and the Greatest Hits appear to have been released on the JIVE label and not PWL so there's that too.
As for singles apparently Tragedy is the highest selling PWL single....but Especially For You isn't too far behind. I Should Be So Lucky and The Loco-Motion were both gold.
Also all the singles from 'Kylie' and I think Hand On Your Heart were certified silver and above (possibly Wouldn't Change A Thing too).
I think it is much closer than i suspected but it seems that the Steps singles charts positions were perhaps not indicative of the sheer large number of copies sold because Kylie's singles largely all out peaked them.