Why does the food not appear to decrease, if the shaytan eats with the one who does not say Bismillah?
Question

With regard to saying Bismillah before eating, if we do not say Bismillah, then why does the food not decrease if the shaytan is eating with us?
Praise be to Allah.It is soundly proven that the shaytan shares a person’s food with him if he does not say Bismillah over it.
Muslim (2017) narrated from Hudhayfah: When we attended a meal with the Prophet (blessings and peace of Allah be upon him), we would not place our hands on the food until the Messenger of Allah (blessings and peace of Allah be upon him) did so first. On one occasion we attended a meal with him, and a girl came (running) as if she was being pushed. She went to place her hand on the food and the Messenger of Allah (blessings and peace of Allah be upon him) took hold of her hand. Then a Bedouin came as if he was being pushed, and he took hold of his hand.
Then the Messenger of Allah (blessings and peace of Allah be upon him) said: “The Shaytan will be able to eat food if the name of Allah has not been mentioned over it. He brought this girl so that he might be able to eat it because of her, so I took hold of her hand. Then he brought this Bedouin so that he might be able to eat it because of him, so I took hold of his hand. By the One in Whose hand is my soul, his hand is in mine, along with hers
And he (2018) narrated from Jabir ibn ‘Abdillah that he heard the Prophet (blessings and peace of Allah be upon him) say: “When a man enters his house, and mentions Allah when he enters and when he eats, the shaytan says: ‘You have no place to stay and no dinner.’ But if he enters and does not mention Allah when he enters, the shaytan says, ‘You have found a place to stay.’ And if he does not mention Allah when he eats, he says: ‘You have found a place to stay and dinner.’”
And he (2033) narrated that Jabir said: I heard the Prophet (blessings and peace of Allah be upon him) say:
“The shaytan is present with any one of you in all his affairs, and he is even present with him when he eats. If one of you drops a morsel, let him remove any dirt that has got onto it, then eat it, and not leave it for the shaytan. And when he has finished let him lick his fingers, for he does not know in which part of his food the blessing is.”
An-Nawawi (may Allah have mercy on him) said:
The words of the Prophet (blessings and peace of Allah be upon him) “The Shaytan will be able to eat food if the name of Allah has not been mentioned over it” mean that he becomes able to eat the food, if the person starts to eat it without saying Bismillah…
Moreover, the correct view, which is the view of the majority of scholars of the earlier and later generations, including hadith scholars, jurists and scholars of kalam (Islamic philosophy), is that this hadith, and similar hadiths which speak of the shaytan eating, is to be understood according to the apparent meaning, which indicates that the shaytan eats in a real sense, because there is nothing rationally impossible about that and there is nothing in Islamic teachings to deny it; rather Islamic teachings confirm that. So it is obligatory to accept it and believe it. And Allah knows best."(Sharh Sahih Muslim  13/189-190).
Once this is established, it is not right for the Muslim to ask too many questions and wonder why the food does not decrease and the like, because how the shaytan eats and his manner of eating are things that are hidden from us, and we are not required to put effort into finding out about something that is beyond our understanding and the revelation has not told us how it is.
Ishaq ibn Rahawayh (may Allah have mercy on him) said:
It is not permissible to think about the Creator, but it is permissible for people to think about created beings on the basis of what they have heard about them; however, they should not add anything to that, because if they do that they will become confused.… Allah says (interpretation of the meaning):
{And there is not a thing except that it exalts [ Allah ] by His praise} [Al-Isra’ 17:44].
So how could it be permissible for any person to try to find out about how clothes hangers and manufactured things glorify Allah, or to try to find out about how plates, bowls, baked bread and woven garments glorify Allah? It is soundly narrated that they glorify Allah, and it is up to Allah to determine how they will glorify Him, in whatever manner He wills. People have no right to indulge in discussion of that except on the basis of knowledge, and they should not speak about it or other, similar issues except on the basis of religious texts, and they should not do any more than that. And Allah is the source of strength, and in Him we put our trust. So fear Allah and do not indulge in talking about these unclear matters, because discussing such issues will divert you from the right path."(Masa’il Harb al-Kirmani  3/1154).
And Allah knows best.