CachedMetdata has a Vector<char> member and calls append() in its constructor. The problem is that Vector::append(data, size) calls expandCapacity(size), which allocates "(size / 4) + 1" more bytes than needed. Metadata can be quite large, 288 KiB was seen in [1], but that is a total number, which decomposes into 230 KiB of actual data and 58 KiB of wasted memory.

The fix is to reserve enough space, either by using Vector(size) constructor, or by explicitly calling reserveInitialCapacity(). Slightly involved fix is to add Vector(data, size) constructor, which might be useful in other places.

There is at least one other place where Vector::append() is called in a constructor: PushMessageData, but I suspect there are instances of this pattern in functions, too. It might be interesting to profile Vector and find out how many instances were not modified after first append().

[1] https://docs.google.com/spreadsheets/d/13knYGLMXB5opE7vlDUsvreNcYr-FEi_mmaJ2YZVfS6M/edit#gid=159723971