So, looking into it, I found no LiPo AAA batteries but I did find LiIon which way about 1/3 to 1/2 of the weight of the typical NiMH ones.
Pack of 5 duracels non-rechargeable alkaline weigh 114g including packaging (home depot). Pack of 5 energizer non-rechargeable Li-Ion weigh 41g (amazon). duracel alk AAA weight/cell = 18-21g (depending on packaging weight) energizer Li-Ion AAA weight = 5-8g (depends of packaging) rechargeable Lion weight = 9-12g (found on Amazon)
These don't state whether they are rechargeable or not but could be worth it if they don't drain too fast just for the light weight consideration: https://www.amazon.ca/Energizer-Ultimate-Lithium-L92BP-4-Battery/dp/B0002DUQDQ
Sunlabz Rechargeable (8-11.3g/cell) https://www.amazon.com/SunLabz-1000mAh-NiMH-Rechargeable-Batteries/dp/B00J18PFIO/ref=sr_1_3?ie=UTF8&qid=1478843835&sr=8-3&keywords=AAA%2Blithium%2Brechargeable&th=1
Rechargeable (6-8.5g) https://www.amazon.com/Docooler-TrustFire-Rechargeable-Lithium-Battery/dp/B01LWLZYG6/ref=sr_1_7?ie=UTF8&qid=1478843835&sr=8-7&keywords=AAA+lithium+rechargeable
Seems to me the lower the capcity, the lighter it's going to be.
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