A user's smartphone loses its charge rapidly.

The user's smartphone is losing its charge rapidly and feels thicker than it was when new. Both of these symptoms point towards a bad battery.

A bad battery loses its ability to hold a charge, leading to rapid draining. Additionally, a bad battery can physically swell, which explains why the phone feels thicker.

Malware infection, damaged charge port, and poor connectivity are unlikely causes for both the rapid draining and the increased thickness.

References

CompTIA A+ Certification Core 1 220-1101 Study Guide, Chapter 2: Mobile Devices, Section 2.2 Mobile Device Batteries

https://www.comptia.org/training/books/a-core-1-220-1101-study-guide