There's a reason why grace notes are printed smaller.
Grace notes are embellishments, not of equal importance as the "larger" note they're connected to.
I hear people all the time playing grace notes as loud or louder than the note they connect to, as if they're important. They're not.
One more time: Grace notes are embellishments! With few exceptions, emphasizing the grace notes is simply wrong.
The only time a grace note may either be louder or of the same volume as the notes they follow is if the style of the music demands that the grace notes fall on the beat, not before it. This is rarely indicated in the printed music; convention has dictated that, even though these notes aren't actually grace notes, they're printed as if they are.
(Some non-European cultures may interpret grace notes differently. If so, I'm ignorant of them.)
In a perfect world (according to me, of course) if notes printed as grace notes are as important as the note they follow, they should printed as "equals" to the connected note and fitted into the beats in the measure, not printed smaller and just tacked on. And if they are to be played ON the beat, they should be transcribed accordingly. However, I don't expect anyone to flout a couple hundred years of convention to satisfy logic.
Listen to various amateur orchestras and bands play the Radetzky March. You'll hear the grace notes played on the beat, off the beat, as loud as the connected note, or as soft as the connected note! (They should be played before the beat. Actually, in this piece they should be played as acciaccaturas. Look it up; I'm not doing all your work for you.)
Listen to all the less-musically-aware players consistently slam grace notes!
All that confusion just because of a printed convention that makes things unclear musically!
So as things are and will continue to be, here are my rules for grace notes:
I always treat grace notes as embellishments, with less emphasis than the note they are connected to.
Unless it is very clear that the musical style dictates otherwise, I always play grace notes before the beat.
If they need to be played on the beat, I still play them as embellishments, with less emphasis.
I never emphasize grace notes that fall before the beat unless the composer is explicit that they should be.
Put the "grace" back in grace notes, don't clobber them!
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