I guess that all those hardware goodies - Snapdragon 810 & 1440p display, maybe combined with Windows 10 eat up more battery than we may expect.
From the official Microsoft Mobile website: Search results for Lumia 950 XL, Lumia 1520 - Microsoft - Global
One interesting thing of note is that under "Other user interface features", the 950 XL has "tactile feedback", which isn't listed under the 1520.
From the official Microsoft Mobile website: Search results for Lumia 950 XL, Lumia 1520 - Microsoft - Global
- | Lumia 950 XL | Lumia 1520 |
Max Talk Time (3G): | 19 | 25.1 |
Max Standby Time: | 12 days | 32 days |
One interesting thing of note is that under "Other user interface features", the 950 XL has "tactile feedback", which isn't listed under the 1520.