The first spider shape an Arachnomancer gets will be a small, medium, and large monstrous spider. Let's just talk about large size for the sake of argument.
According to a National Geographic regarding tarantulas:
Average life span in the wild: Up to 30 years
Size: 4.75 in (12 cm) long; leg span, up to 11 in (28 cm)Weight:1 to 3 oz (28 to 85 g)
According to Wikipedia:
Some of the largest species of tarantula may weigh over 85 grams (3 oz); the largest of all, the goliath birdeater (Theraphosa blondi) from Venezuela and Brazil, has been reported to attain a weight of 150 grams (5.3 oz) and a leg-span of up to 30 centimetres (12 in)
Size Category: Large
Size Modifier: -1
Grapple Modifier: +4
Hide Modifier: -4
Height or Length: 8 ft. - 16 ft.
Weight: 500 lb. - 2 tons
Space: 10 ft.
Natural Reach (Tall): 10 ft.
Natural Reach (Long): 5 ft.
Let's just use the space it takes up; in this case it is 10 ft. So using the statistics for a biggest tarantula, it is 5.3 ounces at 12 inches. Multiplying those values by 10 would leave me at 53 ounces for a 10 ft. long spider. 53 ounces = 3.3 lbs.
Am I doing that math correctly? I am not a biologist, but I think a 10 ft. spider should weigh more than a 6 pack of beer.
Also, I do not believe that a 10 ft. spider should weigh 500 lb - 2 tons.
What exactly is a reasonable weight for a large size monstrous spider? The reason it is important is due to possibly pouncing on people from high heights.