In the case when some font, which is substituted, is used inside one text object with several sizes, there can be incorrect text scaling when size is changed.
Sample PDF enclosed.

It seems that the problem concerns the fonts cache in SplashFontEngine. It uses the same FT_Face instance for all the instances of SplashFTFont of the same font. After you use some new size that not in the cache, the new instance of SplashFTFont sets the size into FT_Face. It works well until you try to use another size which was set for that font before. It is found in cache, but different size in FT_Face has already been set by previous SplashFTFont constructor call and that's a problem. It tries to calculate scale for substituted font in SplashOutputDev::doUpdateFont() and uses incorrect width of 'm' character.

I think, it would be nice to add some activation procedure after it find the font in cache.

To solve the problem I added method 
void SplashFTFont::activate() {
	FT_Activate_Size(sizeObj);
}
and corresponding virtual void activate() {} in SplashFont
and call it after the font is found in cache. It seems that it can solve the problem.