Collaborative power amplifying robots are accepted as one solution to overcome flexibility and ergonomic issues in future work and life scenarios. The handling of various sized and weighted objects in heterogeneous environments pose a particular challenge to the often applied admittance control. Haptic illusions, especially the Size-Weight Illusion (SWI), where the smaller of two equally weighted objects is perceived to be heavier, can have malicious, disturbing, or to some extent useful influence on system stability and usability. A within-subjects experiment was conducted with 40 participants and three within-factors (size, weight, and movement type), to investigate the occurrence and influence of SWI in bimanual fast-imprecise and slow-precise planar manipulation tasks. The illusion was replicated and an influence on usability was found. Further, different control strategies according to object size and mass (static, compensatory, and mismatch) were analyzed and did not show significant effects on task performance. It appears that either no change in assistance or a change according to object size is advisable.