For some reason, some things can take space in the inventory when they shouldn't (medkits and alcohol).
To work around this, you can spend points on the Deeper Pockets skills. All main heroes have this skill in the Survival tree. Each point gives +2 slots.
You can gain +160 slots (more if you're a premium user) by spending 16 points for all 5 main heroes' Deeper Pockets skill.