It's about half as big as a Hot Tub
In other words, 100 gallons is 0.526 times the size of a Hot Tub, and the size of a Hot Tub is 1.9 times that amount.(for Lifesmart Rock Solid Simplicity Plug and Play 4 Person Spa With 12 Jets) (a.k.a. Spa, a.k.a. Jacuzzi, a.k.a. Whirlpool) (Advertised Capacity)
An average, four-person hot tub holds 190 gallons of water. Modern hot tubs became popular outside of Japan starting in about 1940.
It's about two-and-a-half times as big as a Bathtub
In other words, 100 gallons is 2.4 times the size of a Bathtub, and the size of a Bathtub is 0.42 times that amount.(a.k.a. bath, a.k.a. tub) (for Bootz Industries Aloha 5 ft. Left Hand Drain Soaking Tub)
The capacity of a typical bathtub is 42 gallons. Historically, bathtubs have been made of various materials including marble, terracotta, silver, gold, and iron but the size and shape of the tubs has changed little in their 7,000 year history.
It's about five times as big as a Car Gas Tank
In other words, 100 gallons is 5 times the size of a Car Gas Tank, and the size of a Car Gas Tank is 0.2 times that amount.(for 2009 Ford Taurus)
A car gas tank (for example, that of a 2009 Ford Taurus) holds 20 gallons of gas. In contrast, the Curved Dash Oldsmobile — the first mass-produced car — had a fuel capacity of just 5.280 gallons.
It's about six-and-a-half times as big as a Beer Keg
In other words, 100 gallons is 6.45 times the size of a Beer Keg, and the size of a Beer Keg is 0.155 times that amount.(Half barrel; maximum capacity)
The most common keg size sold by US beer sellers is a half-barrel type with a volume of 15.50 gallons. This type of keg contains the equivalent of 165 0.0937 gallons cans of beer.
It's about one-thirtieth as big as a Concrete Mixer Truck
In other words, 100 gallons is 0.03 times the size of a Concrete Mixer Truck, and the size of a Concrete Mixer Truck is 29 times that amount.(a.k.a. cement mixer, a.k.a. cement mixer truck, a.k.a. cement transport truck, a.k.a. concrete mixer, a.k.a. concrete transport truck, a.k.a. in-transit mixer)(standard size; drum capacity)
A concrete mixer truck has a drum volume of up to 2,900 gallons. Concrete mixer trucks can be in transit for about 90 minutes before the concrete dries — if the concrete dries while in the drum, jackhammers or even dynamite must be used to remove it.
It's about thirty times as big as a Basketball (packed)
In other words, 100 gallons is 34.101768487 times the size of a Basketball (packed), and the size of a Basketball (packed) is 0.02932399240 times that amount.(64% packing density) (NBA official ball standards, Size 7)
A NBA official ball, manufactured by Spalding, is a Size 7 ball measures about 1.87673551360 gallons. These balls have 4,118 pebbles each with a diameter of 2.5 mm
It's about 45 times as big as a Bowling Ball (packed)
In other words, 100 gallons is 45.976568523 times the size of a Bowling Ball (packed), and the size of a Bowling Ball (packed) is 0.02175020956 times that amount.(64% packing density) (USBC specifications; tenpin)
According to United States Bowling Congress specifications, a tenpin bowling ball should measure at least 10.795 cm (4.25 in) in radius for a total volume of about 1.39201341150 gallons. Because regulation balls measure the same diameter irrespective of their weight, ball density varies by size and balls weighing less than about 5 kg (11 lbs) will float in water.