In theory yes, but it's not recommended.
You should at least know the basic shape of events in the Belgariad before you start the Malloreon. Fortunately, many (all?) editions of the Malloreon start with a prologue/introduction which summarises the Belgariad. So you can learn that overall shape quite easily, but you'd still be missing out on a lot.
The Malloreon is definitely a continuation of the story of the Belgariad: although the plot is fairly self-contained, a lot of the worldbuilding and character development was done in the Belgariad.
A very brief (mostly spoiler-free) summary is that the continuing narrative through both stories is essentially about a prophesied culmination of a long-running conflict between Good and Evil.
- In the Belgariad, you learn about the prophecy and the people involved, and everything comes to a head in a culmination at the very end.
- In the Malloreon, it's revealed that there's an extension to the prophecy and further events need to happen for an even more final culmination at the very very end.
That's what I mean by the Malloreon being able to function on its own in terms of plot. It has its own narrative arc which is separate from that of the Belgariad, although technically a continuation. As long as you get a brief summary of previous events, you won't feel like you're tossed into the middle of the story.
Worldbuilding is an important part of any epic fantasy like this, set in a completely fictional world with its own countries, continents, and cultures.
- The Malloreon does have plenty of worldbuilding of its own, due to a lot of the action taking place in either Angarak countries or the second supercontinent of Mallorea, but it's pretty much assumed that you're already familiar with the countries and cultures prominently featured in the Belgariad.
- If you read the Belgariad first, you get the common fantasy trope of the main character starting out on a farm with no knowledge of the wider world, so that he can act as an audience surrogate and learn together with you about the cultures of Sendaria, Arendia, the Alorn kingdoms, Nyissa, etc.
So in terms of setting, you would feel tossed into the middle by starting with the Malloreon. You'd need to quickly orient yourself with respect to all the different countries that the main characters already know very well. There's no real audience surrogate in the Malloreon (Errand is arguably a bit of one, but he's not the single main character).
This is strongly related to the last point about audience surrogates, and it's a common issue with series that have a "five/ten/fifteen years later ..." gap in the middle.
- As well as an epic fantasy, the Belgariad is also sort of a coming-of-age story. The main character starts off as a farmboy and you can follow him through his learning curve, not only discovering other countries and cultures but also gaining skills and confidence and learning about himself. Some other characters have revelations making you realise they're not exactly who you thought they were.
- At the start of the Malloreon, all the main characters are grown up and in most cases married with children, although the children aren't old enough to be the new main characters. New Malloreon characters also have some surprising twists (including some of my favourite characters in either series), but the old Belgariad characters mostly remain as we knew them at the end of the Belgariad.
If you start with the Belgariad, you can continue right on with the characters you know and love. If you start with the Malloreon, it might feel like going to a party where everyone knows everyone and you're the only outsider.
Obviously, if you read the Malloreon first, you'll be spoiled on the entire plot arc of the Belgariad. But I'm mentioning this last because I don't think it's a huge issue for two reasons.
Firstly, I seem to recall that you, OP, are one of those people who doesn't care about spoilers.
Secondly, the strengths of this series aren't really in its shocking twists that would be ruined by spoilers. In fact, one of the themes of this universe (a cynic might call it a genius in-universe excuse for some lazy writing) is its repetitiveness and predictableness due to a Problem that can only be resolved by that final culmination. Don't read these books for a thrilling plot; read them for witty dialogue, plenty of larger-than-life characters, excellent geographic worldbuilding, and some really good lead-up to series finales. (Seriously. Eddings prose is so distinctive I could do a convincing imitation of it; his characters are well constructed and memorable; the geography of his world is almost on par with Wheel of Time; and the building of tension before the climax, especially in the Malloreon, is really breathtaking.) Two of those four points would be decreased by reading the Malloreon before the Belgariad.
PS. "(These books are not small.)" Yes they are, says the guy named after the main character of the 4-million-word epic :-)