Album sales could be not part very important in that definition.
https://en.wikipedia.org/wiki/Metallica_discography
2016. Metallica has sold more than
200 million albums worldwide, with over 60 million records in the United States alone (54,365,000 albums since 1991 when SoundScan started tracking actual sales figure).
https://en.wikipedia.org/wiki/Lady_Gaga_discography
As of January 2016, she has sold an estimated
27 million albums and 146 million singles worldwide. She has also sold around 7.25 million singles in the United Kingdom, and 10.4 million albums in US; in the latter country she is the first and only artist to have two songs pass 7 million downloads ("Poker Face" and "Just Dance"). According to the Recording Industry Association of America (RIAA), Gaga is the fourth-best-selling digital singles artist in the United States, with cumulative single certifications of 59 million digital downloads and on-demand streaming.
I would not be surprised if Metallica sold more albums when they had an eight an a half year long discography (this includes
The Black album).
Latest album figures (Metallica's latest album was released one month earlier than Lady Gaga's):
Metallica:
Hardwired... to Self-Destruct
- Released: November 18, 2016
US:
570,000
- RIAA: Gold
- ARIA: Gold
- BPI: Gold
- BVMI: 3× Gold
- MC: Platinum
- RMNZ: Gold
Lady Gaga:
Joanne
- Released: October 21, 2016
- US: 435,000
- FRA: 12,000
- UK: 90,624
Lady Gaga a bigger artist? Well, she is a huge (digital) single artist. Album wise, Metallica is out of her league.
Yes, Lady Gaga is a
pop artist, but Metallica is so big that they dwarf most pop artists.