In the early part of WWII, when escort ships were in short supply, the Brits started building the Flower class corvette, based on a whaler design. Subsequent designs where first improved (revised Flower class, Castle class) and then enlarged (River class). These larger ships were called frigates and designed primarily to remedy what were considered defects in the Corvette design, such as a lack of range, speed, and seakeeping ability. In order to do this, the basic Corvette hull was widened and lengthened, and was given twin screws. This resulted in a faster ship with twice the range.

Note that the Flower class corvette was about the same displacement as ships such as, for example, the WWI Town class destroyer and the 1930s River class destroyers. The River class frigates displaced almost as much as the WWII Tribal class destroyers. Much of the difference between corvettes/frigates on the one hand and destroyers on the others was in their machinery (i.e. in low speed + longer range versus high speed + shorter range) and armament (primarily ASW versus primarily ASuW oriented) . In other words: different role resulted in different armament and machinery for ships of similar tonnage.

Flower class corvette
Original 1939-1940 Programme:
Displacement: 950 tons
Dimensions: 62.5 x 10.0 x 3.5 meters (205 x 33 x 11.5 feet)
Propulsion: 2 fire tube Scotch boilers, 1 4-cyl. triple expansion steam engine, 2750 hp., 16 knots
Range: 3500 nm. at 12 knots
Crew: 85
Radar: SW1C or 2C (later)
Sonar: Type 123A, later Type 127DV
Fire Control:
EW: none
Armament: 1 4″ BL Mk.IX single, 2 .50 cal mg twin, 2 Lewis .303 cal mg twin, 2 Mk.II depth charge throwers, 2 depth charge rails, 40 depth charges.
Originally fitted with minesweeping gear, later removed

River class frigate:
Displacement: 2,216 tons full load
Dimensions: 91.9 x 11.3 x 4.0 meters (301.5 x 37 x 13 feet)
Propulsion: 2 shafts, VTE, 2 boilers, 5,500 ihp, 19 knots
Range: 7200 miles at 12 knots.
Crew: 141
Radar: Type 271, or SU type (varied)
EW: HF/DF (High Frequency Direction Finder)
Armament: 1 4″ single (later dual), 1 12 Pdr, 4 20mm guns, 1 Hedgehog ASW mortar, 4 Mk.IV depth charge throwers, 150-200 depth charges.

Town class destroyer
Displacement: 1,069 (1,190 for BUXTON, ST CROIX, and ST FRANCIS) tons
Dimensions: 95.7 x 9.3 x 2.7 meters (314 x 30.5 x 9 feet)
Propulsion: 28 knots
Crew: 153
Armament: 4 4″ single guns, 12 21″ TT

Tribal class destoyer
Displacement: 2,800 tons full load
Dimension: 114.9 x 11.4 x 3.4 meters (377 x 37.5 x 11 feet)
Propulsion: Parsons geared steam turbines, 2 shafts, 3 Admiralty 3 drum type boilers, 44,000 shp, 36.5 knots (max), 32 knots (service)
Crew: 259
Armament: 3 4.7″ twin mounts (2 forward, 1 aft), 1 4″ twin mount (aft), 1 2 Pdr. quad mount, 6 20mm guns, 4 – 21″ TT