Supposedly the power Escanor has make him to be strong and bulky during day and skinny and weak during night. But in episode 7 of season 3 if I recall correctly, Nanatsu no Taizai members are seen in what apparently is daytime, (there is fog around) and Escanor is skinny.
Short answer: Escanor can remain in his (skinny) nighttime form during the day due to his glasses, a magic item given to him by Merlin which allows him to suppress his powers.
Longer answer: In regards to why Escanor would opt to stay in night form during the day we need to understand him better. Escanor's ability is called sunlight, during the night, Escanor is in his true form and has the lowest power level recorded in the series (15) with a timid and cowardly personality to match but as night turns to day he gradually bulks up and his demeanor becomes arrogant or "prideful" which continues until he reaches a peak for one minute at high noon before slowly weakening until nightfall. While his power is active his physical might and endurance is unparalleled, he can scorch enemies merely by standing near them and generate a pocket sun to fling at people. Escanor was born with this power and has made it known he has despised it his entire life because it has caused him to hurt the people he cares about and be called a monster by everyone else. The power it grants him is immense but is not without drawbacks, his nighttime personality is more or less his actual personality but he cannot suppress the change in persona that comes over him when his power activates. In addition, while the ability is powerful it was never intended for humanity to wield,
Sunlight is the power of Mael, one of the 4 Archangels of the Goddess clan whose power somehow manifested in Escanor
as a result, Escanor drains his lifespan due to the strain put on him by the power. Finally, he's supposed to be laying low, it is made apparent in Chapter 148 when Jericho stumbles into Escanor's bar that he hasn't gotten the news that the sins' name has been cleared of their supposed crime from 10 years ago. When Garlan knocks Escanor out and his glasses off only to find his sunlight powered form the next morning it allows Meliodas and Merlin to easily track him down by detecting his overwhelming aura, a task that would've been drastically quicker if he'd been activating sunlight everyday.
I could not find the chapter the magic glasses were explained, although in chapter 162 right before Gloxinia and Dolor's tournament Gowther lands on Escanor shattering his glasses which causes him to shriek that 1. they were a gift from Merlin and 2. without them he... (he gets impaled by Gloxinia before he completes his sentence). The wiki describes his magic glasses as a gift from Merlin which when removed in daytime causes his sunlight to instantly activate. He later gets a new pair of glasses off-screen and it should be noted he only appears in nighttime form during the day while wearing his glasses.