she does not sing glinda the way she sings her own records, and the difference is a deliberate, technical, hard-to-do piece of work.
the short answer
she sings glinda in a light, high, forward soprano placement with a fast narrow vibrato — a legitimate musical theatre sound rather than her pop voice, which sits lower and breathier. the part also requires comedy delivered inside the singing rather than around it, which is the harder half of the job.
the placement is different, on purpose
her pop records sit low, forward and breathy, with the weight of the arrangement coming from stacked harmony rather than from volume. glinda is the opposite: high, bright, unbreathy, with an audible fast vibrato and none of the pop-vocal habits — no scooping into notes, no melisma, no falling off the ends of lines.
that is a full change of technique rather than a change of key. singing a role rather than singing as yourself is the first thing musical theatre teaches, and it is what her actual training consists of — a florida children's theatre from eight and broadway at fourteen.
the comedy is inside the line
the influence she named years earlier
kristin chenoweth, the original broadway glinda, is somebody grande had named as an influence long before the film existed — and the two of them worked together on hairspray live! in 2016. the light-soprano-with-comic-timing tradition glinda sits in is chenoweth's, and grande knew it thoroughly before she was asked.
sung live on set
jon m. chu has confirmed that she and cynthia erivo both insisted on singing live during filming rather than miming to studio pre-records. that constrains everything above: a placement you can only hold in a booth is no use on a set, and the comic timing has to survive being acted at the same time.
it is also, by a reasonable argument, why defying gravity won a grammy in a category full of contemporary pop singles two years later.
what to listen for
play popular and then play pov. the same person, four years apart, using two entirely different instruments. then play the girl in the bubble, written for the second film, where glinda's voice has to carry something the character has never had to sing before.
questions people ask
does ariana grande sing differently as glinda?
yes. glinda is a light, high, forward soprano with a fast vibrato and no pop habits. her own records sit lower and breathier.
did ariana grande sing live in wicked?
yes. the director has confirmed she and cynthia erivo both insisted on singing live on set rather than miming to studio recordings.
what makes popular hard to sing?
the comedy is inside the vocal line rather than between the lines, and keeping the technique correct while doing that is much harder than either alone.
who is kristin chenoweth to this?
the original broadway glinda, an influence grande named years before the film, and her co-star in hairspray live! in 2016.
which songs show the difference best?
play popular against pov. the same singer four years apart, using two entirely different instruments.