There isn't a universally accepted standard for what counts as bulkier, at least in competitive. EVs are a major factor in deciding bulk.
In Showdown!, bulk is calculated by taking the product of the HP along with Def and SpD to find out their physical and special bulk. Simply speaking, the higher the product is, the tankier/bulkier the Pokemon is without going into any mechanics of damage calculation.
Taking on the example of Hariyama and Dusknoir, the former has high base HP (144) but middling defenses (60 and 60), meanwhile Dusknoir has much superior base defenses (130 and 130) but suffers from low HP (45).
At no defensive EVs investment, Dusknoir has a slightly higher product total and therefore takes somewhat less damage.
.bulk hariyama (evs: 0/0/0)
undefined bulk (evs: 0/0/0)
physical: 66,924 [429 hp, 156 def]
special: 66,924 [429 hp, 156 spdef]
(bulk is hp times defense)
.bulk dusknoir (evs: 0/0/0)
undefined bulk (evs: 0/0/0)
physical: 70,686 [231 hp, 306 def]
special: 70,686 [231 hp, 306 spdef]
(bulk is hp times defense)
252+ SpA Kyogre Ice Beam vs. 0 HP / 0 SpD Dusknoir: 93-110 (40.2 - 47.6%) -- guaranteed 3HKO
252+ SpA Kyogre Ice Beam vs. 0 HP / 0 SpD Hariyama: 181-214 (42 - 49.7%) -- guaranteed 3HKO
As the difference between their products isn't very much, the damage rolls are nearly the same.
Maximum HP but no defense EVs
.bulk dusknoir (evs: 252/0/0)
undefined bulk (evs: 252/0/0)
physical: 89,964 [294 hp, 306 def]
special: 89,964 [294 hp, 306 spdef]
.bulk hariyama (evs: 252/0/0)
undefined bulk (evs: 252/0/0)
physical: 76,752 [492 hp, 156 def]
special: 76,752 [492 hp, 156 spdef]
252+ SpA Kyogre Ice Beam vs. 252 HP / 0 SpD Dusknoir: 93-110 (31.6 - 37.4%) -- 87.1% chance to 3HKO
252+ SpA Kyogre Ice Beam vs. 252 HP / 0 SpD Hariyama: 181-214 (36.7 - 43.4%) -- guaranteed 3HKO
(a 5% damage roll difference compared to 2% earlier)
In this case, when both have only max HP EVs, the Pokemon with lower HP but higher defenses is better at tanking hits than the one with already high HP but low defenses. The reason is pretty simple that HP EVs do not have much of an impact on a Pokemon whose base HP is already high, but can make a huge difference for Pokemon with low base HP.
Maximum defense EVs but no HP
.bulk dusknoir (evs: 0/252+/252+)
undefined bulk (evs: 0/252+/252+)
physical: 93,555 [231 hp, 405 def]
special: 93,555 [231 hp, 405 spdef]
.bulk hariyama (evs: 0/252+/252+)
undefined bulk (evs: 0/252+/252+)
physical: 102,960 [429 hp, 240 def]
special: 102,960 [429 hp, 240 spdef]
252+ SpA Kyogre Ice Beam vs. 0 HP / 252+ SpD Dusknoir: 70-83 (30.3 - 35.9%) -- 42.2% chance to 3HKO
252+ SpA Kyogre Ice Beam vs. 0 HP / 252+ SpD Hariyama: 118-139 (27.5 - 32.4%) -- guaranteed 4HKO
The opposite is also true. A Pokemon with low defenses but high HP has greater returns on investing in defenses compared to a Pokemon with high defenses but low HP.
Maximum HP and defense EVs
.bulk dusknoir
undefined bulk (evs: 252/252+/252+)
physical: 119,070 [294 hp, 405 def]
special: 119,070 [294 hp, 405 spdef]
.bulk hariyama
undefined bulk (evs: 252/252+/252+)
physical: 118,080 [492 hp, 240 def]
special: 118,080 [492 hp, 240 spdef]
252+ SpA Kyogre Ice Beam vs. 252 HP / 252+ SpD Dusknoir: 70-83 (23.8 - 28.2%) -- 93.8% chance to 4HKO
252+ SpA Kyogre Ice Beam vs. 252 HP / 252+ SpD Hariyama: 118-139 (23.9 - 28.2%) -- 94.8% chance to 4HKO
When both of them have maximum HP and defense EVs investments, the difference in the products is negligible. As such, both take nearly the same damage.
So, it can be concluded that EVs along with their base stats are important in deciding bulk. In case of Hariyama and Dusknoir as we've seen, they have almost same product of HP and defense so they have nearly the same "bulk".
If both have maximum defense investments, then the one with greater product of HP and defense can be considered tankier. However, one thing to note is that since EVs are limited and you can't invest 252 in both defenses alongside HP, a Pokemon with high HP is generally left weaker on one side.
0 Atk 0 IVs Kyogre Liquidation vs. 252 HP / 0 Def Dusknoir in Rain: 93-109 (31.6 - 37%) -- 79.6% chance to 3HKO
0 Atk 0 IVs Kyogre Liquidation vs. 252 HP / 0 Def Hariyama in Rain: 180-213 (36.5 - 43.2%) -- guaranteed 3HKO
Hence, a Pokemon with high defenses would be able to tank better on the defensive side EVs aren't invested in.
But as BananaBro said, high HP Pokemon take moves like Seismic Toss and Night Shade easier thanks to their larger HP pool. The demerit is that opposing Pokemon will also heal more from Leech Seed.
This is all from a mathematical viewpoint though. In practical use, many high base HP Pokemon leave their HP untouched and focus on both defenses (eg. Blissey, Alomomola running 4 HP / 252 Def / 252 SpD). Also note that when finding out a good bulky Pokemon raw stats are not the biggest deciding factor in competitive. Several other factors like ability, typing and moves have a greater impact than raw stats alone. There are many examples where Pokemon such as Uxie, Probopass, Regice, etc. have been not so great in competitive for many generations despite having high defensive stats. Meanwhile, as sumwun mentioned, Pokemon like Clefable, Gliscor, Zapdos, Landorus-T have been defensive staples when their bulk is only average compared to the defensive Pokemon mentioned earlier.
Source: PMing .bulk [Pokemon] (evs: a/b/c) to Scrappie, a bot in Pokemon Showdown! and damage calculator
