I did get the "equal" bit.
Not all combat sports have weight classes ! I like watching Sumo because you can often enough see a small guy win from a much bigger and heavier guy. You can also see that some have experience in Aikido. You will find that in many if not most combat traditions there is no such thing as weight classes. Even you would do research in older combat systems, as I have, again you will not find weight classes in those classic and sometimes ancient systems. Weight classes are a modern Western development in an effort to keep the match exciting. From a warrior or Budo point of view it has little meaning.
As for the example of ikkyo; now you are talking about skill - experience - understanding. And by that I mean not just your skill - experience - understanding as shite, but also that of your training-partners as aite. If done well it is not likely that aite can get up once he has been brought down with ikkyo. But getting up in order to try to lift shite is not a valid counter; it is way to dangerous for aite.
There are valid counters for ikkyo, but they do not involve muscle strength or the supposed advantage of weight. I suggest you ask your sensei about ikkyo and its valid counters.
In Aikido there is really no need to make up for a "strength deficit". If we would need that, then what would be the sense of calling it Aikido?
I disagree. This is a valid way to counter ikkyo, if the technique is done poorly. Okay, it's not really aikido, but it does effectively point out when nage is doing a technique poorly.