Font A has a UPM value of 256 and Font B has a UPM value of 1024.
(Both are TTF files.)
When I move the glyph from font A to font B, the size of the letter is imported into a very small state.
So I set the UPM value of Font A to 1024, and then moved it, but the size was still imported small.
Why doesn’t the value of “Font Info > Font > Unit per Em” actually apply to glyph even if I modify the value to 1024?
How can I increase the actual UPM size of a font with a UPM value of 256 to all 1024?
There are two ways of changing the UPM of a font.
The first is typing a new value into the UPM field in the Font Info window. This simply redefines the UPM, but keeps the coordinates of the glyph outlines in the font unchanged. If you increase the UPM value, for example, then the outlines shrink relative to this new UPM.
Instead, you want to scale the font to a new UPM. This changes the UPM value, but also scales all coordinates in the font to match the new UPM. If the new UPM is not a multiple of the old UPM, you may get slight rounding errors in the coordinates. But 1024 is a multiple of 256, so no rounding errors will occur.
Scale a font to a new UPM using the scale 
button next to the UPM field. See the Handbook for more details:
You might need to play around with the UPM values a bit to get the relative size you like (by scaling to smaller or bigger UPM).
And you need to regenerate your composite glyphs: Diacritics | Glyphs
