Blend

As is the key with all Aikido techniques, stealing the attackers balance by accepting the attack and allowing them to overextend is how you develop softness of technique. If you find that you are struggling against the weight or strength of your partner, then you are not going "deep" enough, ie. you are not accepting the attack deeply and blending with the attack deep enough. This shallow reaction is natural in the beginning. Do not let it frustrate you. Just become aware of it and focus on entering deeply almost to the point of exaggeration. This will help you naturally embody a deep blend with your opponent.