Never done bike packing but is weight as critical as it is when on foot as the bike takes the weight.
To a degree, yes. But terrain is also a factor. If it's a fairly flat route and most climbs are gradual, then weight doesn't matter. If it's hilly/mountainous, weight is a bigger factor, and hopefully your chosen gearing will compensate. For bikepacking trips, my lowest gearing is 19", which will get me up most things, but will still have a bit of hike a bike (especially if I'm underbiked for the given terrain).
As mentioned above, size is a bigger factor and my biggest struggle. I ride a 46cm (extra small) bike, so space is at a premium, and therefore a lot of my stuff packs up small. But I still have a lot of bags compared to someone who has a larger bike. I can fit a max of 4 litres in between my handlebars- so that is either my sleeping bag (which I don't like to compress that much but will for an overnighter) or my tent (either, but does not include any poles) with some electronics crammed in, the rest are in my top tube bag. My repair kit lives in my frame bag, and is on the bike permanently. I can fit my phone and some gels/small snacks in there as well, but not much else. My sleeping kit (minus sleeping bag) is on one fork, cook kit and first aid/toiletry kit on the other (i tend to take my firemaple for anything more than an overnighter), poles and food in the cargo pack, sleeping bag on one side in a 10 litre pannier, spare clothes (cycling shorts, sleeping clothes, socks) in the other. I could fit more in my cargo pack and lose my fork bags, but then I lose the ability to use my dropper post and most of the weight is centred towards the rear of my bike and I prefer to keep it more balanced. Rain jacket is shoved in the front pocket of my handlebar bag for ease of access. Water bottles are on the downtube and in 1 x stem bags, the other stem bag has my water filter and easy access snacks.
If I'm only off for an overnighter, and I know the terrain (and know I won't need my dropper post), I wil just use the cargopack, frame bag and handlebar bag as I won't need an extra set of cycling kit, will take the alcohol stove, and shove my sleeping bag in my handlebar bag and everything else in the cargopack.
When I take my son, the big guns come out bag wise (plus stuff strapped everywhere)- 22 litre panniers, 10 litre fork bags- he's only 4, so I need to carry everything (except for some toys- those go in his little saddlebag and handlebar bag). But it's flat, so doable.