Well, as a former powerlifter who stills trains with weights regularly (though somewhat more mildly), I can tell you that what physical bulk I have has not been a particular hindrance to my Aikido. I was told by a yondan once that I would never reach shodan if I continued to work out at the gym. Presently, I am closing in on sandan.
The only trouble that i have in Aikido from weightlifting is very stiff shoulders. When sensei applies nikyo osae it is very unpleasant. I am working to develop greater flexibility in this joint, but it is slow going.
I would wholeheartedly recommend weightlifting. If I'm going to be impeded by bulk i'd rather it was muscle than fat!