: Heavy weight vs Light weight? My goal is to build muscle size. There is lot of information online which says like Arnold says(3rd point) you need to lift heavy to increase muscle size and there's other information as well

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Muscle size increases when exercise causes microtears in your fibres, and your body recovers. The repair process strengthens the muscles by increasing its size.

You can microtear your muscles both ways (heavy and light), although the standard rep scheme for bodybuilding is 3x8 (heavy enough to tear effectively, and light enough that you can tear multiple times).

PPL is a popular weightlifting routine to build muscle size (https://www.google.ca/search?q=ppl+routine&oq=ppl+routine&aqs=chrome.0.0l6.2447j0j7&sourceid=chrome&ie=UTF-8).

Also, your body releases more anabolic hormones when you lift heavy, while it releases catabolic hormones when you lift light.

All in all, you can't go wrong with 3x8. (under 6 reps taxes more your CNS [central nervous system] than tears your muscles - for example, at 8 reps, each rep tears the muscle by 0.1%, while at 5 reps each rep tears the muscle by 0.12% (20% increase), you tear 3 x 0.8% = 2.4% with 3x8, and only 3 x 0.6% = 1.8% with 3x5).

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You want to increace the size of your muscles, this process is called muscle hypertrophy and it involves an increase in size of skeletal muscle through a growth in size of its component cells.

Two factors contribute to hypertrophy: sarcoplasmic hypertrophy, which focuses more on increased muscle glycogen storage; and myofibrillar hypertrophy, which focuses more on increased myofibril size.

Generally speaking, the effects of exercise are thus:

1-6 reps (>85% 1RM) of work is generally accepted/acknowledged towards building Myofibril Hypertrophy (actual muscle fiber hypertrophy). Also increases overall strength (which influences #2 and #3 ) [1]
6-12 reps (70-85% 1RM) is work is generally accepted/acknowledged towards building Sarcoplasmic Hypertrophy (tissues surrounding the muscle fibers). [1]
12+ reps (<70% 1RM) develops mitochondria and fatigue resistance of a muscle, particularly Type I, predominantly aerobic fibers. Also allows you to tolerate more total work capacity. Note: Lower-intensity, longer-duration aerobic exercise generally does not result in very effective tissue hypertrophy; instead, endurance athletes enhance storage of fats and carbohydrates within the muscles [2]

So matter of fact, it is not better or worse to lift light or heavy, they just lead to slightly different outcomes.

Based on the information above, my recommendation is to lift a wide variety of rep ranges from high intensity (which means low reps, high weight) to medium intensity (traditional 8-12 body building approach) to light intensity (endurance fiber training/fatigue resistance up in reps of 15-50 in some cases) at various times in your program.

TL;DR:
Just lift, both light for high reps and heavy for less reps.

Note, the principles behind muscle growth are still debated

[1] Kraemer, William J.; Zatsiorsky, Vladimir M. (2006). Science and practice of strength training. Champaign, IL: Human Kinetics. p. 50. ISBN 0-7360-5628-9.

[2] van Loon LJ, Goodpaster BH (2005). "Increased intramuscular lipid storage in the insulin-resistant and endurance-trained state". Pflügers Archiv - European Journal of Physiology 451 (5): 606–16. doi:10.1007/s00424-005-1509-0. PMID 16155759.

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You'll need to do both to some extent, and as always it depends on context.

Notes on the articles linked

Firstly the Arnold article also mentions he recommends doing higher reps at a lighter weight (8-12 reps) for some exercises so the "going heavy" recommendation isn't absolute.

Secondly the VERDICT in the Flex article is an example of misconstrued conclusions. The study involved doing 4 sets of 4 on the leg extension, compared to 4 sets of around 25. 4x4 on leg extensions is useless for testing the effect of heavy weights, no one does that in real life. What we can take from this article is an indicator about the effectiveness of high rep sets on isolation movements (legs particularly) for building muscle, but we probably can't take away information on the effectiveness of heavy reps. Regardless, the SENTENCING part at the end is solid and as you can see promotes a variety of rep ranges.

So frankly the solution is to do a variety of rep ranges.

Sarcoplasmic vs Myofibrillar hypertrophy is a fuzzy area in current literature, I wouldn't bother worrying about it. What we do reliably know is that volume (how much work you do, more sets or more reps at more weight is more volume) is the primary driver of hypertrophy.

What should I do?

Do the bulk of your muscle building work in the 8-12 rep range as it's been repeatedly shown to allow for the most work in an efficient time period in a safe manner.

Do some of your main compound lifts lower (3-8 reps) and push for strength on them.

Do some of your isolation work higher (above 12 reps to as high as you like) for the size building benefits this rep range offers too.

Always bear in mind regardless of this: none of your lifting should be "easy". Whether heavy or light you are going to want to push yourself on every set to give your body a reason to grow. If you're just coasting gym workouts you won't get anywhere. You don't need to kill yourself in the gym either, but find that balance.