This study aims to evaluate the accuracy of prosthesis size prediction in MAKO robotic-assisted total hip arthroplasty (RATHA) and to assess the impact of discrepancies between the preoperative robotic plan and the actual sizes of femoral prostheses applied during surgery on the postoperative stem position. We conducted a retrospective analysis of preoperative planning, intraoperative records, and postoperative imaging data from patients who underwent RATHA. We predicted prosthesis sizes using both traditional 2D methods and robotic systems, comparing their accuracy. The cases were categorized into four groups based on the consistency between the robotic planned and applied femoral sizes: matched group, reduced group, enlarged group, and deviated group. The postoperative measurements of stem position were compared across these groups. The prediction accuracy of the robotic system for the acetabular cup (90.3% vs. 69.7%, P < 0.01) and femoral stem (75.7% vs. 59.5%, P < 0.01) was significantly higher than that of the traditional 2-D method. The absolute values of coronal plane angulation (CA) varied significantly among the three groups: 0.55° (0.15°-1.08°) in the enlarged group, 1.25° (0.40°-2.40°) in the matched group, and 2.55° (1.45°-3.13°) in the reduced group (P < 0.001). The safe zone ratio was higher in the enlarged (100%, P < 0.01) and matched groups (83.6%, P < 0.01) compared to the reduced group (60.7%). A larger stem was associated with a higher canal fill ratio (CFR) in planes C (P = 0.02) and D (P < 0.01). All data from the deviated group differed significantly from those of the other three groups. The robotic system provides a more accurate prediction of prosthesis size compared to traditional methods. The use of a smaller stem than planned was linked to a larger absolute CA and a smaller CFR. Conversely, applying a larger stem facilitates a more neutral stem position and improved press fitting. If the applied size is reduced by two or more compared to the postoperative plan, poor positioning of the femoral prosthesis should be suspected.