Apologies if this has been dealt with before.

A medium sized character gains a +10 modifier against a one-step bigger opponent (hulking) and a -10 against a one-step smaller opponent (scrawny). Modifiers basically increase the bigger the opponent is and decrease the smaller he is.

So would a scrawny character get a +10 against a medium sized opponent? And +20 against a hulking opponent?

I think this is absolute rather than relative values ​​. At least when it comes to shooting. Use different modifiers for melee weapons and firearms... I think it will be too complicated .

Apologies if this has been dealt with before.

A medium sized character gains a +10 modifier against a one-step bigger opponent (hulking) and a -10 against a one-step smaller opponent (scrawny). Modifiers basically increase the bigger the opponent is and decrease the smaller he is.

So would a scrawny character get a +10 against a medium sized opponent? And +20 against a hulking opponent?

This is an interesting question, and one I've thought about myself a number of times.

Jargal is correct in making the statement it becomes complicated.

What I first did was consider the to-hit modifiers based on Size as absolutes (as Jargal suggests), but then toyed with modifiers to Dodge and Parry based on Attacker/Defender Size...and it got out of hand very quickly. In short, an Average Attacker vs an Average Defender did not modify Dodge or Parry. For Parry, I assigned a modifier if the Defender was one or more Size categories smaller or larger (-10 per category). That kind of makes sense- a Space Marine attempting to Parry the attack of a Heirodule seemed far less likely to succeed than parrying the attack of an Ork Nob, or a Gretchin. For Dodge, I assigned static modifiers based on the creatures Size- a Human (Average) Dodged at +/-0, while a Servo-Skull (Puny) Dodged at +20. But then there were such considerations as Power Armour being an enhancement, not a hindrance, or Quadruped, and I finally decided to throw the whole Dodge concept in the garbage.

I kept the Parry concept (which was, I believe, first introduced in No_1's 'Aprocrypha Somethingerother Tyranids'. It highlights the distinct differences between Dodge and Parry, making Players think a little more about their Advancements, and further defines a PC's combat niche.

If you're interested in going part-way to addressing your concern about this seeming disparity, I say keep Size modifiers to hit as absolutes, assign a modifier to Parry based on size difference, and if you're not happy with that you can always go the complicated route.