Previous work on Social Comparison Theory shows that comparing oneself to others can lead to negative self-perceptions and rumination, reducing self-confidence. Despite these harmful effects, social comparisons are frequently used as engagement strategies in personal informatics systems, such as health and wellness apps. There is limited understanding of how users perceive these comparisons and their impact on wellbeing. To address this, we reviewed the Top 50 Health & Wellness smartphone applications to analyse implemented comparison strategies and the metrics such comparisons are used for. We conducted an online vignette study (n=192) and an interview study (n=12) to further explore the impact of social comparisons on users. Our study shows that comparisons in personal informatics motivate users but simultaneously lead to negative emotions (e.g., inferiority, disappointment), potentially leading to obsessive thoughts and overtraining. Based on our findings, we propose design guidelines for implementing social comparison features that prioritise users' wellbeing.