The DC65 features reconfigured brushbar that can pick up more dirt, it is full size while the DC40 is mid size, therefore it has wider cleaning path, larger bin, longer cord, and a bit heavier than the DC40.

Difference Between Dyson DC65 and DC40

	Dyson DC40 Animal	Dyson DC65 Animal
Images
Cyclone Technology Type	Radial Root Cyclone™ technology	Radial Root Cyclone™ technology
Ball Technology	YES	YES
Design	Mid Size Upright	Full Size Upright
Bin Capacity	0.42 gallons	0.55 gallons DC65 has larger dust bin.
Weight	14.6 lbs DC40 is lighter than the DC65.	17.35 lbs
Suction Power	200 airwatts	245 airwatts DC65 provides more powerful suction.
Multiple Floor Types Cleaning	YES	YES
Bagless	YES	YES
Corded	YES	YES
Cord Length	24.6 ft	35 ft DC65 has longer cord.
Max Reach	40 ft	51.7 ft
Cleaner Head and Brushbar	Self-adjusting, Motorized brush bar	Self-adjusting, Reconfigured brushbar, 25% more power drives bristles deeper into carpets removing more dirt.
Cleaner Head Width	about 12.2"	about 13.3″
Warranty	5 year warranty on parts & labor	5 year parts and labor warranty
Dimensions(H x W x D)	41.9" x 14.3" x 12.2"	42.4" x 13.4" x 15.4"

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One of the major difference between them is their sizes. Both of these 2 Dyson vacuum cleaners are upright, however, Dyson DC65 is full size while the DC50 is compact. Since DC65 equip with more powerful motor, its noise level is higher.
Because of the bulkier size, the DC65 features larger bin and wider cleaner head, and it has more powerful suction.
The brush bar on Dyson DC65 is reconfigured, it can pull out more dirt and dust of the carpet.
The DC50 is lighter and easy to carry.
The hose on DC50 is also shorter.

Dyson DC65 vs DC50

	Dyson DC50 Animal	Dyson DC65 Animal
Images
Cyclone Technology Type	2 Tier Radial™ cyclones	Radial Root Cyclone™ technology
Ball Technology	YES	YES
Design	Compact Upright	Full Size Upright
Multiple Floor Types Cleaning	YES	YES
Suction Power	128 airwatts	245 airwatts
Bagless	YES	YES
Bin Capacity	0.21 gallons	0.55 gallons
Corded	YES	YES
Cord Length	24.9 ft	35 ft
Max Reach	28.5 ft	51.7 ft
Cleaner Head Width	about 11″	about 13.3″
Cleaner Head Technology	Self-adjusting	Self-adjusting
Warranty	5 year warranty on parts & labor	5 year parts and labor warranty
Dimensions	30.1" x 11" x 13.9"	42.4" x 13.4" x 15.4"
Weight	11.6 lbs	17.35 lbs

Overall, the DC50 is good for small space while the DC65 is for people that need a more powerful vacuum cleaner for cleaning larger house.

If you know other and difference between Dyson DC50 and DC65, please share below.

According to the reviews from customers who have both DC41 and DC65, Dyson DC65 has more suction power than the DC41, it picks up hair from carpets and fine dust better than the DC41. for some fine particles, DC65 only require one pass to pick up while the DC41 have to go 2 passes.
The Dyson DC65 feature the reconfigured brush bar that can help remove more dirt, dust and allergens from your home.

Dyson DC65 vs DC41

	Dyson DC41 Animal	Dyson DC65 Animal
Images
Cyclone Technology Type	Radial Root Cyclone™ technology	Radial Root Cyclone™ technology
Ball Technology	YES	YES
Design	Full Size Upright	Full Size Upright
Multiple Floor Types Cleaning	YES	YES
Suction Power	235 airwatts	245 airwatts
Bagless	YES	YES
Bin Capacity	0.55 gallons	0.55 gallons
Corded	YES	YES
Cord Length	35 ft	35 ft
Max Reach	51.7 ft	51.7 ft
Cleaner Head Width	about 13.3″	about 13.3″
Warranty	5 year warranty on parts & labor	5 year parts and labor warranty
Dimensions	42.4" x 13.4" x 15.4"	42.4" x 13.4" x 15.4"
Weight	17.1 lbs	17.35 lbs
Cleaner Head Technology	Self-adjusting	Self-adjusting

If you know other similarities and difference between Dyson DC65 and DC41, please share below.