Druid spider animal companion advancement

[Ryshin]

Hello, one of my player went for the feat "spider companion" that allows him to get monstrous spiders as animal companions.

He sustain that he can via feat get a medium size spider (2HD) and with the bonus HD obtained from the druid table increase the actual size of the companion to huge when it gets to 8HD gaining for example the +16 bonus str and +8 bonus con from the monster advancement table from monster manual in addition to the bonus str and dex from druid table, and the size of the spider companion will keep increasing it size accordigly to the bonus HD so it will reach gargantuan size at 16HD. He sustain that he can increase the actual size because in the moster manual there are every version of monstrous spider from tiny to colossal.

I as DM on the other hand argue that any druid animal companion gets bonus HD by druid table which grant him bonus str, dex, nat armor, ecc but it won't actually increase the size, so no additional stat bonuses. To increase the size of his companion a druid must change the companion when he reach then required HD and get a new one of a bigger size from the list, so right now he can either have his medium size spider companion with 8HD and improved stats or a new huge spider companion.

I furthermore argue that since the spider companion feat grant him only spider companions from tiny to huge, he can increase the HD of said spider but he won't be able to increase the size to gargantuan when it reaches 16HD so it will always be at best a huge spider.

Thanks

Spider Companion
( Drow of the Underdark, p. 52)

[General]

Instead of an animal, you bond with a monstrous spider

Prerequisite
Vermin Trainer (DrU) , animal companion,

Benefit
You can select a monstrous spider as your animal companion. This spider gains all the benefits normally granted to animal companions. At 1st level, you can select a Tiny, Small, or Medium monstrous spider as your spider companion. A druid of sufficiently high level can select a more powerful creature as her spider companion, applying the indicated adjustment to the druid's level (in parentheses) for the purpose of determining the spider companion's characteristics and special abilities.

Spider   Minimum Level (Adjustment)
Large   4th(—3)
Huge   10th(—9)

Normal
Spiders are not available as animal companions.

[Skyrock]

An article from the old D&D website explicitely rules out size increases for animal companion HD: https://web.archive.org/web/20210709145229/http://archive.wizards.com/default.asp?x=dnd/rg/20070206a

Bonus Hit Dice.
The animal companion gains extra Hit Dice as noted in the table on page 36 of the Player's Handbook. The companion gains the full benefit from increased Hit Dice, including increased base attack, base saves, skill points, and feats. The animal companion, however, does not increase in size (any more than you do when adding Hit Dice for your class levels).

PF is different and grants some animal companions size increases (e.g. shark): https://legacy.aonprd.com/coreRuleBook/classes/druid.html#animal-companions-184

[Ryshin]

Thanks! This is very helpful, my player is gonna hate me though