Saving filesizes to disk doesn't happen

Nobody has claimed this yet.

I can see there's an option `writeFile` and it is set to true by default, and a file named `size-plugin.json` should appear where sizes are being written to. Such a file never appears for me
and when I rollup my sources (without the watch option) it always looks like there were no changes to last run:

I tested in Nodejs 12.10.0 as well as 13.2.0.

Here's an excerpt of my rollup config (I have tried to specify an explicit file name there, but both relying on the default and using an explicit file name fail):

```js
export default [
{
input: 'src/api.mjs',
output: [
// config for
{
file: pkg.module,
format: 'esm'
},
// config for <script nomodule>
{
file: pkg.browser,
format: 'umd',
name: 'Cookies',
noConflict: true,
banner: ';'
}
],
plugins: [licenseBanner]
},
{
input: 'src/api.mjs',
output: [
// config for <script type="module">
{
file: pkg.module.replace('.mjs', '.min.mjs'),
format: 'esm'
},
// config for <script nomodule>
{
file: pkg.browser.replace('.js', '.min.js'),
format: 'umd',
name: 'Cookies',
noConflict: true
}
],
plugins: [
terser(),
licenseBanner, // must be applied after terser, otherwise it's being stripped away...
size({ filename: 'sizes-min.json' })
]
}
]
```

Start by reproducing the issue with the shown Rollup configuration and the writeFile option set to true. Trace where size-plugin.json and the explicit sizes-min.json filename are handled, then verify that a file is created and records changes between non-watch builds.

Written by the indexing model from the issue text.