In short, the amount of 1.5 Kg is for each person. So in total you would distribute 15 Kg to 10 people.
The expiation is to feed ten poor persons, 1.5 KG of rice for each poor person approximately.
It is equivalent to approximately one and a half kilograms of rice, which should be given with something that can be eaten with it, to one poor person. It is obligatory to give to ten different people. If you can find a family of poor Muslims composed of ten members, or two or three families who numbers add up to ten, and you give them fifteen kilograms of rice, along with some meat, for example, this will be sufficient.
The fact that 1.5Kg is too much for a person to eat in one serving is irrelevant. That is because the madhab that you are acting on does not assume the verse عشرة مساكين من أوسط ما تطعمون أهليكم (feeding of ten needy people from the average of that which you feed your own families ) to mean just one meal.
In detail, the amount of food that is sufficient of Kaffarah is disagreed upon by the scholars. The amount of food that suffices for Kaffarah according to the major madhabs is as follows:
Two complete meals to the same person at different times (e.g. lunch and dinner) such that they eat enough to fully sate their bellies. The person must be an adult with an empty stomach. In other words this is equal to entirely satisfying the requirement of food of a person for an entire day, since average people ate two meals in a day. OR
Staples such as grains, flour or fruit. The amount to be given is the same as that for sadaqat al-fitr: one saa' for dates, barley and other staples but half a saa' for wheat.
The Hanafis also allow giving the price of the quantity of food to the poor person.
- By giving staples such as grains, flour or fruit etc. The amount to be given is one mudd of wheat or two mudd of dates or barley or others
- By giving staples such as grains, flour or fruit etc. The amount to be given is one mudd.
By giving two meals, similar to the Hanafis. OR
By giving staples such as grains, flour or fruit etc. The amount to be given is one mudd
References: Badaa’i’ al-Sanaa’i’, Al-Mughni Ibn Qudamah , Rawdat at-Taalibeen, Mawaahib al-Jaleel
How much is a saa' or a mudd in metric units?
Firstly know the interrelation between the two units. 1 saa' == 4 mudd (and hence 0.5 saa' == 2 mudd). This is agreed upon.
There is difference of opinion in the quantification of these measures. Further they are units of volume so conversion into weights varies with the density of the foodstuff. A mudd is the amount that can be held in two hands with fingers extended. In very rounded off terms the volume of a saa' is said to be around 3 Liters and by weight it is said to be around 3-4 Kg.
Islamqa and Islamweb in the above linked answers have taken the value of the mudd to be 750 grams, which makes half a saa' equal to 1.5 Kg.
Most of the Hanafis take the value of the saa' to around 4 Kg making half a saa' to be 2 Kg or 1.7 Kg of wheat.
There are also other estimates, e.g. taking a mudd to be 500-680 grams.