The Hub's "medium" instance size refers to c1.medium (High-CPU), not the m1.medium (High-Mem). We'll be adding support for m1.medium in the next round of Hub updates. 

For reference:

M1 Medium Instance

- 3.75 GiB memory
- 2 EC2 Compute Unit (1 virtual core with 2 EC2 Compute Unit)
- 410 GB instance storage
- 32-bit or 64-bit platform
- I/O Performance: Moderate
- EBS-Optimized Available: No
- API name: m1.medium

High-CPU Medium Instance

- 1.7 GiB of memory
- 5 EC2 Compute Units (2 virtual cores with 2.5 EC2 Compute Units each)
- 350 GB of instance storage
- 32-bit or 64-bit platform
- I/O Performance: Moderate
- EBS-Optimized Available: No
- API name: c1.medium