small notehead in a chord

[MING TSANG]

lilyponders:

For the following code, how can I make notehead Ef, Ef, in the chords  smaller?

\version "2.19.7"

\language "english"

{

\key ef\major

\time 3/4

\clef bass

<ef, ef af>2. |<ef, ef g>2.

}

Emmanuel,

Ming.