roundtripping fontc-built Oswald's cmap table with ttx mysteriously halves its size

I'm still not sure why

```
$ git clone https://github.com/googlefonts/OswaldFont
$ cd OswaldFont
$ fontc -s sources/Oswald.glyphs
$ ttx -q -o - build/font.ttf | ttx -q -o build/font#1.ttf -
$ ls build/*.ttf
build/font#1.ttf build/font.ttf
$ diff -u <(ttx -q -o - -t cmap build/font.ttf) <(ttx -q -o - -t cmap build/font#1.ttf)
$ echo $?
0
```

So far no diffs in the high-level XML content after roundtripping through ttx, but if I compare the cmap table sizes before and after ttx, I get:

```
$ diff -u <(ttx -l build/font.ttf | grep cmap) <(ttx -l build/font#1.ttf | grep cmap)
```

```diff
tag checksum length offset
---- ---------- -------- --------
- cmap 0x8D57590D 6212 30844
+ cmap 0x90BB83AA 2392 4036
```

The fonttools generated cmap is less than half the size of the fontc/write-fonts generated one.. Why?