Weight and size aren't even a consideration and haven't been for me bar a few rare occasions, Now I'm not a weight lifter nor am I all that strong, basically if you need it you'll put up with the weight and size, if its too heavy you won't buy it then you really don't need it. In simpler terms if you can find a reason to complain about an item to the point where you won't buy it then you never needed that item in the first place and the excuse is really irrelevant. If you really do need the item you'll find some way of owning it regardless of weight and size.
The only 2 times i can think of where weight and size came up for me are when i sold my recently acquired F*600mm F4.0 which is 15lbs for a Sigma 500mm F4.5 which is 6.8lbs ... yes i could justify giving up 100mm and 1/3rd of a stop for a drop of 8.2lbs of weight. before that i didn't know where my upper limit on weight was. Now the F*600mm F4.0 was still usable it was just taxing after 2 hours in the bush handheld and i didn't car to use a tripod or monopod when in the bush. More recently while evaluating the Tamron (Clone) 15-30mm F2.8 over my Sigma 8-16mm F4.5-5.6 I decided that the Tamron at over double the weight and larger didn't really give me enough in return to require dealing with the extra, sure 2 more stops of light is impressive but so far I've demonstrated i can get away with doing without. Cost (trade-up difference) played a roll too. I'm not denying its a great lens, I'm just understanding that i don't need it. In all other lens and camera related purchasing size and weight have never played a roll.
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It is more a case of weighing (no pun intended) ALL of the Pros and Cons of moving to a particular system.
FF lenses are inherently larger than their APS-C counterparts.
For the same resulting photo they are not.
The DA*50-135mm F2.8 should be compared to a 70-200mm F4.0... you'll find very little difference in size, A 16-50mm F2.8 should be compared with a 24-70mm F4.0.
However just to throw a wrench into things the Rokinon 16mm F2.0 APS-C lens is far far larger than its 24mm F2.8 FF counterparts.
Yet even holding the DA* 50-135mm lens for a few miles walk in your hand and you'll surely notice it. Now consider adding a few more pounds for a FF body and 70-200mm lens. The difference becomes even more noticeable as the focal length increases. There is a reason (keeping with my example) the DA* 50-135mm does not have a mono/tri-pod mount and all of the 70-200mm lenses do. And it isn't to make them look professional.
No not all of them do... the 70-200mm F4.0's which are identical in all image parameters give you match to total image noise to the 50-135mm F2.8 on APS-C. Please don't make the mistake of thinking F2.8 on APS-C is the same as F2.8 on FF, if you're going to convert 1 value in a comparison you have to convert all values or you're comparing apples to oranges
If you don't see it as an issue, that is your opinion and your view. But it is an actual consideration to make when moving systems.
For example a 70-200mm F4.0 on FF will proved the same end result as a 50-135mm F2.8 on APS-C and a 35-100mm F2.0 on 43rds and a 25-75mm F1.4 on 1" and a 17.5-50mm F0.7 on 2/3rds" and baring differences in actual length they will all have the same size front element and all be around the same weight.
The 70-200mm F2.8 on FF will always have a 1 stop advantage over the 50-135mm F2.8 on APS-C and therefore is 1 stop larger and therefore an unfair comparison.
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