The answer hinges on whether or not magical items that use the "gain the "enlarge" effect of [spell]" are considered as having applied the spell.
Sage Advice says it Dispel Magic only works on spells
The 2019 Sage Advice compendium (page 14) states the following:
Can you use dispel magic to dispel a magical effect like a vampire’s Charm ability or a druid’s Wild Shape?
Dispel magic has a particular purpose: to break other spells. It has no effect on a vampire’s Charm ability or any other magical effect that isn’t a spell. It also does nothing to the properties of a magic item. It can, however, end a spell cast from a magic item or from another source. Spells—they’re what dispel magic is about. For example, if you cast dispel magic on a staff of power, the spell fails to disrupt the staff’s magical properties, but if the staff’s wielder casts hold monster from the staff, dispel magic can end that spell if cast on the target of hold monster.
There are abilities and other spells that can end or suspend magical effects that aren’t spells. For example, the greater restoration spell can end a charm effect of any sort on a target (such as a vampire’s Charm or a dryad’s Fey Charm), and a paladin’s Aura of Devotion can prevent or suspend such an effect.
Extrapolating this, it would seem that, unless the actual spell itself is used (ie cast), the effect is not dispellable. However, a DM may house-rule that such effects are still dispellable.
However, the examples used in the Sage Advice Compendium entry deal with abilities that produce an effect but without referencing a spell. That is, the Vampire's Charm ability does NOT say "as though under the effect of the Charm spell" or something similar.
Some spells take this approach. For instance, Potion of Flying reads:
When you drink this potion, you gain a flying speed equal to your walking speed for 1 hour and can hover.
This potion does something very similar to the spell Fly but does not use the "effect" wording (and allows for hovering).
Because it does not say "gains the effect of a Fly spell," or reference the spell in any way, it would be treated like the vampire's charm ability. To wit: it would not be dispellable.
But Potion of Enlarge does reference a spell and says it produces that spell's effect:
you gain the "enlarge" effect of the enlarge/reduce spell
Dispel magic reads:
Choose one creature, object, or magical effect within range.
Potion of Enlarge's description seems specifically tailored to make it susceptible to Dispel magic.
I think it has to use this wording because it could not say "When consumed, it casts [spell] on the imbiber." That sort of wording would allow Counterspell to be used against it and prohibit Barbarians from drinking health potions while raging. That just doesn't seem right, hence the wording that Potion of Enlarge uses.
I think the general rules for using magic items to cast spells applies here, too:
Some magic items allow the user to cast a spell from the item. The spell is cast at the lowest possible spell level, doesn't expend any of the user's spell slots, and requires no components, unless the item's description says otherwise.
While the rule specifically references items that cast a spell, I think it is not at all a stretch to assume that it applies to any magical item that produces an effect of a spell, whether that's from a spell cast or a simple statement that the user benefits from the effect.
So the combination of these rules suggests that if the wording of the magical item states that the beneficiary has "gained an effect of a spell," it is to be treated the same as though the spell itself had been cast and is therefore dispellable by Dispel Magic. The DC for doing this is treated as though the spell were cast/applied at the lowest level.