Output size difference is always the same size as actual output

Hi,

Love the idea of this plugin ! Has anybody got it working ?

For me the output of the plugin is always the same the actual file size output, i.e. it never compares with previous builds

I.e. sample output

```
vendors~admin~create-bet~login.bundle.9aa89c670076013d8fb7.js ⏤ 20.9 kB (+20.9 kB)
main.css ⏤ 2.18 kB (+2.18 kB)
main.bundle.ca78aa7c8d6b80d6a794.js ⏤ 8.28 kB (+8.28 kB)
2.css ⏤ 107 B (+107 B)
dashboard.bundle.c2606a256910c68c213f.js ⏤ 1.17 kB (+1.17 kB)
create-bet.bundle.3b2e0767f6308ec7e4ca.js ⏤ 1.23 kB (+1.23 kB)
4.css ⏤ 205 B (+205 B)
admin.bundle.5740df303ba85130515a.js ⏤ 3.02 kB (+3.02 kB)
login.bundle.96773d2bb3d6c67df42a.js ⏤ 1.04 kB (+1.04 kB)
vendors~main.bundle.73fbc0215d9b89852b15.js ⏤ 149 kB (+149 kB)
index.html ⏤ 541 B (+541 B)
```

And here is my sample webpack config :

https://github.com/stevenfitzpatrick/mybettingslips/blob/master/build-utils/webpack.prod.js

And yes I tried to change the order of the size plugin putting it first / last always same result.