Exactly Mobile Testing is more complex than of Web Testing. Let me give the use case and it will be easier to analyse the complexity of Mobile App Testing -
1. Mobile Phone Screensize dependency
2. Mobile Phone OS (Android version 5, 6,7,8,9) dependency
3. Mobile Phone RAM dependency
4. Mobile Phone Memory dependency
5. Mobile Network dependency
6. Mobile Screen rotation (horizonatal / vertical)
7. Mobile different resources allocation (Camera/File Explorer) dependency
8. Different interrupted testing like - Call/SMS/notification
These all cases must be catered in Mobile Application Testing, where as Web application does not have these cases, hence Mobile application testing is much complex than of Web application testing.