In OpenD6 there are tables defining how much weight you can lift with the weightlifting skill. I wonder though: is there any info in all of D6 on whether size also affects the max. weight you can lift? For example, if you are bigger by +5 size classes, how would that change things?
That's a tricky question: scale and size rules are somewhat ambiguous in the different rule sets.
The base OpenD6 system (rulebook, p. 70-71) only mentions scale as the factor influencing rolls between objects or beings of different sizes, and even then only in the context of combat. Scale is measured in dice (eg. a 5D Ogre fighting 3D scale humans creates a 2D difference). The smaller attacker gains the difference in scale dice as a bonus to hit since its opponent is so much larger, while the latter gains the same bonus as damage dice. However, no mention is made, as far as I can tell, about regular skill rolls outside of combat.
Official D6 system implementations
Now, implementations of the system such as D6 Fantasy treat scale differently (using a flat bonus instead of adding dice) but using similar rules. From the D6 Fantasy rules (see p. 71, or the SRD, Scale section):
For most cases, use these rules: If the attacker is larger than the defender, then the scale modifier is added to the combat difficulty and the damage total. If the attacker is smaller than the defender, then the scale modifier is added to the attacker’s combat skill total and the defender’s damage resistance total.
The wording is different but, again, the rules only mention of damage and hit difficulty.
Unofficial D6 systems
I had a look at some different systems based on OpenD6 in case they could bring some alternative scale rules, without much luck.
Azamar is built on Cinema6, a somewhat simplified OpenD6 system, so I had a look at its scale rules (p. 145) but it's similar to Fantasy D6 in that attackers gain a flat Damage bonus while defenders get a flat Dexterity bonus.
Again, in Mini Six, scale rules deal with combat only (p. 6):
When dealing damage to a smaller scaled target or resisting damage inflicted by it, the larger adds the difference in modifiers to their die rolls or static defenses. Smaller scaled targets attempting to dodge attacks from or actually attempting to attack a larger scaled opponent add the difference to their dodge and attack rolls.
Since there doesn't seem to any official or alternative rules concerning rolls outside of combat at various scales, I'll allow myself an interpretation of the rules.
Considering that, in combat, damage is a measure of how much the attacker affects the defender, it seems reasonable to rule that, outside of combat, the same principle would apply, so a +8 scale Giant with 3D Lifting skill has an above average skill for a creature of its size: a Giant of average strength would obviously still be much, much stronger than a human who's also of average strength (a +8 flat bonus being, on average, just above a +2D bonus).
That makes sense since Attributes, for instance, are limited to 5D, whether you're a snail or an intergalatic battlecruiser. So, in a certain sense, scale is the one means to differentiate averageness among different beings.