Poor Twilight, first she lift 2 tons TOM for Rarity in Discord episode and now this, I wonder why she wont just levitate him ( she can levitate 50 tons giant bear, so why she even bother to use her physical strength, or maybe she just do it to stay in good physical shape ).
Should not she learn it in Pinkie Keen episode where she was smashed and crushed all over the place or in Discord episode where she was hurt by her friends and even had to lift 2 tons TOM for Rarity, poor Twilight, she is ready to endure so much for her friends ( sometime she not even use her powerful magic when it would make everything so much more easy ).
Actually Twilight became Alicorn and grown a bit in season 3 finale, so as much as she would still have to work hard to carry Spike around ( if she wont just use magic or make Spike walk himself ), she should be little bigger and have wings ( not that it change much, unless you count Twilight getting a bit of Earth Pony strength as Alicorn ).
Poor Twi. I see a very painful fall in her future... I suppose it is her fault in a way for not encouraging Spyke to lead a healthy lifestyle when he was young enough to buy into it. "Get ripped to impress Rarity" and all that
No need to worry, if Spike will get to heavy, Twilight can always use her magic to lift him ( she maybe have trouble to lift 2 Tons TOM with strength alone, but with her magic she can lift 40 tons Ursa Minor, so yea, Twilight is strong enough to handle it ).
Yes it would, but what challenge would it be to lift Spike with magic, and considering that Twilight rarely go out of library, lifting big size spike can be good excessive as long as she have strength to lift him, and once it will be to much, Twilight will just switch from strength to magic and problem solved.