I noticed a slight difference in the size of glyf tables compiled by fontTools and those compiled by other font editors (such as FontLab, for example).

fontTools-compiled glyf tables are usually smaller in size.

I believe the difference has to do with the fact that fontTools does not pad glyphs data to 4 byte boundaries, as specified in the loca section of the OT specs:

the local offsets should be long-aligned, i.e., multiples of 4. Offsets which are not long-aligned may seriously degrade performance of some processors.

In the table__g_l_y_f.compile method, extra padding only takes place for odd-lengthed glyphs, which are rounded up to make them fit in the "short" version of the loca table (provided they don't exceed the max size supported by the latter).

So I guess the assumption here is that such performance issues referred to by the specs are no longer relevant given current processors?